The 7-day injured list (known as the 7-day disabled list until the end of the 2018 season) is specifically for players with concussion symptoms.
Players may be placed on the 7-day injured list "retroactively," meaning the stint is backdated to the day after the last date on which the player appeared in a game. For instance, if a player is diagnosed with a concussion on May 12 but last played on May 9, he could be placed on the 7-day injured list on May 12, retroactive to May 10. In that case, he would be eligible to return from the injured list on May 17. A 7-day injured-list stint cannot be backdated by more than five days.
After the 2018 season, the 7-day disabled list (or "DL"), 10-day disabled list and 60-day disabled list were renamed, becoming the 7-day injured list, the 10-day injured list and the 60-day injured list.
Before the creation of the 7-day DL in 2011, there were only two iterations of the injured list: a 15-day version and the 60-day version. Major League Baseball instituted the shorter version to prevent a player with concussion symptoms from being rushed back before he is ready, while also allowing a player who passes concussion tests to return after only seven days rather than 15. The decision to change the 15-day DL to the 10-day DL after the 2016 season did not affect the parameters of the 7-day DL.
The Tigers placed shortstop Jose Iglesias on the 7-day injured list on April 21, 2017, retroactive to April 20, with a concussion. Iglesias was activated April 27, 2017 -- the first day he was eligible.