In 2014, the size of practice squad rosters was increased from eight to ten, and a change was made to the practice squad eligibility requirements that allowed for two out of those ten practice squad roster slots to be filled by players with no more than two accrued NFL seasons toward free agency, regardless of how many regular season games they had been game day actives. For 2016 and 2017, the NFL and NFLPA have agreed to increase that number from two out of ten to four out of ten.
That basically means that any NFL player who was drafted or signed as a college free agent in 2014, 2015, or 2016 is potentially eligible for one of those four out of ten practice squad slots, since none of those players could possibly have more than two accrued NFL seasons. The other 6 slots can only be filled by players with no more than eight regular season games on a game day active list during their accrued season or seasons. So in theory, 2014 first round draft pick Bradley Roby would be potentially practice squad eligible, but of course he would first have to be waived and clear waivers, and there's less than a snowball's chance in hell of that happening.
Since some may be unfamiliar with the term "accrued season," here's a brief explanation. Basically a player accrues an NFL season for each season that he spends at least 6 regular season games on the regular roster of 53 or on IR.
If a player has fewer than three accrued seasons when his contract expires, he becomes an ERFA. Three accrued seasons would make him a RFA, and four or more would make him an UFA (with the exception of players on the 5th year option, or on transition or franchise tags).