Around the same time that the Bengals placed Dre Kirkpatrick on the Active/NFI list, Cincinnati also announced that cornerback Nate Clements (abdominal), Jordan Shipley (knee), Pat Sims (ankle) and Mike Nugent are headed to the Active/Physically Unable to Perform List. Unlike the Reserve/PUP, any one of those players can return to practice as soon as they're medically cleared.
Clements has been dealing with an abdominal problem throughout the offseason with some wondering about the extended time his recovery is taking. Pat Sims is also recovering from a pair of ankle injuries last year that placed him on Injured Reserve, missing the final five games last season. Jordan Shipley is also rehabilitating from a major knee injury during the second game of the season last year while Mike Nugent's injury isn't immediately known.
If those players need an extended period of time to recover, then the Active/PUP is the first step towards the reserve/PUP where players sit out for the first six weeks while the Bengals receive a roster exemption for that player.