The Inactive List is another term for Injured Reserve. If a player is designated as on the IR in the NFL, that player is eligible to be placed on the Inactive List in the DFL. Players on the Inactive List do not count toward the salary cap, but are also not eligible for games.
Once a player is removed from the IR in the NFL, that player must be removed from the Inactive List in the DFL, either by promotion to the Active Roster, traded, or released.