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.
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 injured list 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 injured list to the 10-day injured list after the 2016 season did not affect the parameters of the 7-day injured list or the 60-day injured list. MLB brought back the 15-day injured list specifically for pitchers and two-way players prior to the 2020 season. This also had no affect on the 7-day injured list or the 60-day injured list.