Mission Slots

 determine how many settlers assigned to each job can be deployed on missions.
 * A free slot is required for each settler deployed on a mission, with missions that deploy multiple setters requiring multiple free slots.
 * You can increase the number of mission slots available to each job by making settlements successful, and by specializing them into communities.
 * Some jobs have mission slots unlocked by ranks of the Trailblazer perk.
 * Each rank of Trailblazer (7 in total) is unlocked at 1/3/6/10/15/21/28 Successful Settlements.