Absences can have two effects on how blocks are built. If the absences have credit, they may mean that fewer pairings are needed to construct the block. Absences that don’t have credit can make a complete block more difficult to construct because a complete block must be created using a smaller pool of pairings and days available.
For example, a Crewmember with a single week of vacation will need fewer pairings to construct a complete block. However, a Crew member with 21 days of vacation may be more likely to be unblockable if there are no pairings left in the pool during the remaining days of the bid period in which he/she is available.