Running back Christian McCaffrey was drafted April 28 by the Carolina Panthers.

He reported to the Panthers' practice facility May 4 for a three-day rookie minicamp.

He signed his first professional contract later that day, a four-year deal with an option for a fifth that will guarantee him about $17 million.

He practiced with his new team for three days, and then departed and made plans to return -- in a month.

Yes, McCaffrey is affected by an NFL rule that prohibits rookies from fully participating in offseason programs until their school's spring academic calendar is complete. As a result, McCaffrey -- who attended Stanford -- will miss most of the Panthers' offseason practices.

The rule, part of the NFL's player personnel policy, vexes coaches and players who are eager to get started. So what's it all about? Let's take a closer look.

What exactly does the rule state?

According to the league, a rookie's eligibility is determined by the conclusion of exams at his school, whether or not he is enrolled. For 2017, if exams end before May 15, the player can participate fully. If they wrap up on or after May 15, he is ineligible for all but rookie minicamp until after the final day of exams.

What if the player has withdrawn or otherwise isn't enrolled in school when he is drafted?

Sorry. He is still bound by the rule and ineligible until the last day of exams for students who are enrolled.

What about players in graduate school?

If a player graduated the previous year and played his final college season as a grad student, he is eligible regardless of his school's academic schedule.

So what does that mean for a player such as McCaffrey?

According to the Panthers' website, McCaffrey is not enrolled in school during this quarter. Per the Stanford registrar, the final day of exams there is June 14. The Panthers' final offseason event is their veteran minicamp, scheduled for June 13-15. McCaffrey told reporters in Charlotte that he would return in early June. Based on current information, he would be eligible for the final day of minicamp.

Who does this impact?

Primarily, those who played at schools that use the quarter system and thus have final exams and graduations in June. Most of them are located on the West Coast. Stanford, UCLA, Washington, Oregon and Oregon State are the most prominent. Northwestern also is in this category. I counted 16 drafted players from these schools in 2017 from a total of 253 draftees. That includes first-round picks Solomon Thomas (Stanford/San Francisco 49ers), John Ross (Washington/Cincinnati Bengals) and Takkarist McKinley (UCLA/Atlanta Falcons). Undrafted rookies fall under the same policy.

What's the point?

In conjunction with the NCAA, the rule in theory removes any incentive for a newly signed rookie to leave school early to participate in a team's offseason program.

But don't many players withdraw after their final college season to focus on draft prep?

Yes. That's a primary reason many coaches, players and agents believe the rule is outdated.

Then why doesn't the NFL eliminate it?

It might one day, but some players still remain in school, so for them the rule is relevant. In either event, it is not a high priority on the NFL's agenda.

Why not?

While coaches gripe and players get antsy, the rule is more an annoyance than a career-changer. Let's face it. At the urging of the NFL Players Association, the NFL scaled back its offseason program in the 2011 collective bargaining agreement. As a result of the rule, rookies can miss only 13 possible days of programming before training camp -- a maximum of 10 for organized team activities and three for veteran minicamp. Many teams schedule fewer than 10 OTAs.

But don't rookies stand to benefit from it more than veteran players?

Yes, but the damage is minimal. Even though teams install concepts and plays during offseason practices, almost all teams restart that process on the first day of training camp. The biggest adjustment for rookies who miss the offseason program is assimilating to the speed of an NFL practice. Meanwhile, ineligible players are permitted to speak with coaches on the phone or online and via Skype or FaceTime (among other applications) under the guidelines of the CBA.

Huh. You don't seem too worried about this thing.

Oh, I think teams and players would benefit if the rule were eliminated. It isn't serving the purpose it was designed for as much as it once did. But I don't think it's a threat to the republic. You see some hand-wringing about it every year at this time, and then the narrative quickly is forgotten. You would have to work hard to find a football person who can trace an unresolved player deficiency to this rule.