An accrued season is any season that a player is on a team’s roster (both active and inactive), Injured Reserve or Physically Unable to Perform (PUP) list for more than six games. The above lists qualify as full pay status for an NFL player, and six games under full pay status count as an accrued season.
Accrued seasons are used in a variety of ways within the NFL, notably in calculating the type of free agent a player is when their contract expires. A player with less than three accrued seasons is considered an Exclusive Rights Free Agents (ERFA), a player with three accrued seasons but less than four is considered a Restricted Free Agent (RFA), a player with four accrued seasons (a vested veteran) is an Unrestricted Free Agent (UFA) when their contract expires.
Any player who does not spend the required six games under full pay status does not acquire an accrued season. One notable example of this is former San Francisco 49ers running back Marcus Lattimore. After being drafted in 2013 out of South Carolina following a major knee injury, he spent his entire first season in the league on the non-football injury list, which is not under full pay status. As such, 2013 did not count as an accrued season for him, essentially making 2014 his rookie year.
When players are on the non-football injury list or a practice squad, games for their club do not count toward full pay status and the acquisition of an accrued season. Per the 2011 Collective Bargaining Agreement (CBA): Any player with no accrued seasons is considered a rookie, and once a player has acquired an accrued season they are considered a veteran.
Accrued seasons can also count toward practice squad eligibility. Any player with less than one accrued season is eligible for an NFL practice squad. In addition, all NFL teams can have up to four players with no more than two accrued seasons on their practice squads.
Finally, accrued seasons differ from credited seasons in that credited seasons require only three games at full pay status to be counted as a season. Credited seasons are used to calculate pension eligibility, as well as the minimum salary the player is required to make based on how many credited seasons they have played in the NFL.