If it's an injury deal, it's really a unique situation. If it's a short-term thing, they'd carry Moore on the roster; he's played 43% of defensive snaps so far, according to Reiss.
If it's a long-term thing, the difference between this and IR is (a) the player probably gets paid less on the PS (depending on split contract status and/or the Pats going over PS minimum to retain him) and (b) Sterling Moore retains practice squad eligibility for next season, should the Pats want to keep a player they've already had for nearly a year, and given a substantial snapcount to, for further evaluation.
It's plausible, but hardly the only explanation. Also plausible is this speculative shot-in-the-dark: this is an Alex Silvestro type-deal, where the Pats have seen enough of Sterling Moore at his "natural" position to think the player doesn't project well there as an NFL player, but like some skills or intangibles of the player enough that they want to work him at another position for a while to see if he can improve and break through there.
Sterling Moore seems to be a decent tackler for a corner, and he seems to have good ball skills and a heady demeanor, to the extent he can break up plays even after being soundly beat by a faster player. His problem seems to be he's slow for a corner. What do they call slow, physical, ball-hawking corners? Safeties. They tried him there last year and he didn't look comfortable. So, they don't think he'll improve at corner, and they've already got one developmental safety on the roster with more ST value (Nate Ebner, excluding punt team), so they put Moore on the practice squad as they attempt to transition him to safety.
