PTO - This is a 25 game contract with an AHL team, of which a player can only be signed to one twice per season. This allows a player to play for the AHL only. If they are released to the ECHL, the contract is ended.
ATO - ATOs are usually signed at the end of the season with players who are coming out of college or major junior. Often, these players are draft picks of the affiliated NHL club and will join the club to get a headstart on what will be their team for the coming season. ATO's do not expire after a certain number of games, but if they go to play in the NHL or any other league, they must be officially released from the ATO first.
Essentially, PTO's are seen as almost having a standard player contract and that you are a permanent fixture on the team. ATO's mean you are earning a spot on the team. As Carr was joining the team as a college student ending his career and looking to earn a spot on an AHL team next year, he is an amateur trying out for a contract.
If you've played professional hockey you get a PTO, if you haven't you get an ATO.
