The Advanced button exposes the bottom portion of the dialog. That area contains two frames, Candidate Filters and Availability (Insert Assignment) or Replace During (Replace Assignment).
This frame is used to filter the list of candidate resources. This frame is disabled in the Insert Assignment dialog when Skill or Non-Labor candidates are shown. The frame is always enabled in the Replace Assignment dialog because the candidates are always resources.
If a Primary Skill is selected, the only candidates listed will be those with that Primary Skill. The Secondary filter can be used to find resources with a skill or set of skills regardless of whether the skill is Primary or Secondary.
The settings in this frame are used to modify the time period during which availability is calculated and to filter the candidate list to those resources who have availability greater than or equal to a specified amount.