You don't have to be a soccer business wonk to have heard of Soccer United Marketing, the commercial arm of Major League Soccer that handles sponsorship and marketing deals for many high-profile soccer events in the United States.

But even the most keen observers of the industry have often struggled to shine light on SUM's inner workings - especially the revenues generated by the events SUM puts on.

Those revenues have long been considered by outsiders to be pretty big, as SUM holds or held rights relating to the U.S. men's and women's national teams, the Mexican men's national team's games in the U.S., the Gold Cup and the Copa América Centenario.

The U.S. women's national team's place in SUM's portfolio has been in a particular spotlight recently, because the women's players' union has been trying to bolster their case for equal pay by citing the revenues their immense popularity has generated.

On Wednesday, that spotlight grew significantly brighter.

Two U.S. senators - Dianne Feinstein (D., Calif.) and Patty Murray (D., Wash.) - wrote a letter to MLS commissioner Don Garber asking him to provide details of how much money SUM gets from the U.S. men's and women's national teams, and how that money is used by the organization.

It might not be coincidental that the two senators hail from states where women's soccer is immensely popular. It certainly isn't coincidental that they are longtime advocates for women's rights and gender equality.

Murray and Feinstein write, in part:

According to U.S. Soccer financial reports, SUM pays a yearly minimum amount to U.S. Soccer in exchange for the commercial rights to the Men's and Women's Teams. Additional revenue generated from marketing U.S. Soccer events beyond a specified amount is split between SUM and U.S. Soccer. In 2014 and 2015, SUM paid $15.4 million and $18.3 million to U.S. Soccer respectively under this arrangement which includes $6.7 million and $5.5 million in revenue sharing. However, given the pooled contract and sales structure, it is unclear how much of either the $15.4 million or 18.3 million is attributable to the Women's or Men's Teams alone.

Additionally, SUM owns the rights to leagues and conferences beyond just U.S. Soccer, including the men's professional soccer league MLS, the Mexican National Team, and several other leagues and conferences in the Americas. When negotiating with sponsors, SUM may bundle sponsorship of the Men's and Women's Teams with a variety of other soccer teams and tournaments in your portfolio. Given the fact that many details of marketing agreements are confidential, this adds to the confusion about the revenue generated for U.S. Soccer that is attributable to either the Women's or Men's Teams.

This general lack of transparency surrounding the revenue generated for U.S. Soccer by SUM makes it challenging to accurately understand the magnitude of the disparity in the revenue generated by the Women's and Men's Teams and the impact that differentials in these revenues have on the salaries of the Men's and Women's Team players. In order to clarify these issues, we request that SUM provide:

1. A breakdown of the annual revenue generated for SUM by the Women's Team and the Men's Team over the past eight years.

2. A description of the individual revenue streams that include revenue generated from the Women's Team over the past eight years.

3. The percent of contracts that include sponsorship of Women's Team games.

4. A description of any ongoing efforts by SUM to promote the growth of women's soccer both domestically and abroad.

5. Any other information you deem critical to help us better understand the relationship between the disparity in compensation and the marketing rights to U.S. Soccer.

You can read the full letter at the bottom of this post.

A spokesperson for Soccer United Marketing told me that the organization is in the process of preparing an official response to Sens. Feinstein and Murray, and won't be commenting further until the response is delivered.

How much legal force does the senators' inquiry have? I was as curious as many of you are, so I reached out to a few sports law experts for some insight.

One was UCLA professor Steven Bank, who you may know from his commentary on Twitter about the intersection of soccer, business and legal matters.

Bank told me that because the senators' letter is not a formal subpoena, SUM can politely decline the request if it so chooses. But it might not want to do that.

"If SUM simply refused to comply, it would risk the Senators seeking a formal Committee investigation and a subpoena," Bank said. "That requires some process, but these Senators could certainly raise that as a legitimate threat. As a result, I expect SUM will provide a response of some kind, although the fact that this is already the subject of an active EEOC investigation (I believe it's still active) will likely color the response."

The Equal Employment Opportunity Commission investigation Bank referred to is the complaint over pay inequality filed by the women's national team players' union against the U.S. Soccer Federation.

While SUM's response would not be subject to the Freedom of Information Act, the senators would not be required to keep it confidential. Similarly, there's nothing stopping SUM from making its response public.

Bank noted that the separation of money pots that Sens. Feinstein and Murray hope to expose may not actually exist.

"It may indeed be impossible to separate out the infomation if the deals are pooled in a way that doesn't assign a value to each individual part," Bank said.

U.S. Soccer assigns something of a value to each part in its annual budget, which includes details on revenues and expenses for the various national team programs. It publishes those figures because it is a sport governing body, and as such is technically a non-profit entity (notwithstanding the amount of money it generates).

Soccer United Marketing is a separate company - emphasis on the word company. It's also a private enterprise, so it's not subject to the same disclosure requirements that are required of public corporations.

But SUM is certainly subject to a lot of public scrutiny. Marc Edelman, a law professor in the business school at Baruch College in New York, didn't hesitate to call SUM and MLS the same entity, and said the legal system might have the same perspective.

"In so many areas of law, courts will look at substance over form," he said. "From my perspective, the distinction between Soccer United Marketing and Major League Soccer is relatively impertinent in this context."

MLS also doesn't enjoy the kind of historical popularity that other American leagues do, especially the National Football League and Major League Baseball. MLS probably doesn't have as big a lobbying budget either.

But that doesn't necessarily give Congress the benefit of the doubt.

"Congress historically has liked formal investigations of sports, even if at the end of the day Congress has a track record of doing very little," Edelman said. "It gives them the opportunity to hobnob with people that they find interesting."

That confirms the original question this matter poses: How much of the senators' inquiry is principle and how much of it is public relations?

We'll find out in the days and weeks to come.

The Twitter handle above is for my general news reporting. My soccer handle is @thegoalkeeper. Contact me there for any questions about this post.