If you have people you want to schedule only when all your other people are unavailable, you can get creative and use a combination of Needed Positions and Tags to set this up. This is especially helpful if you want to use Auto-Scheduling in the Matrix, but you want it to only choose your primary people.
Setup ALL People Using Tags
On the left of the main people page, create a tag for each Position and then assign ALL applicable people to that property (Primary and Backup). Click and drag that tag onto all the applicable people.
Assign Primary People Directly to Positions in your Service
From any plan in your service edit your Team and make sure to choose "Assign People to Positions Directly"
Next edit your positions in that Category and ONLY add your Primary people for that position. Don't add any of your backup people. If you plan to use Auto-Schedule from the Matrix, it will schedule anyone listed in this box, so we are excluding the alternate people.
Schedule Your Primary People
When scheduling, click on "1 person needed" to do your regular scheduling and see your list of primary people.
Since these primary people are the only ones technically setup for this service position, when you use Auto-Schedule, it will only choose these people for this position.
Schedule Backups When Needed
Hopefully, scheduling backup people shouldn't be a regular occurrence. So when you need to, skip the needed positions box, and click "Schedule a Person". Then instead of typing a person's name, type the name of the Custom Property you created that lists ALL your people.