Once a volunteer has been assigned to a position, if you want them to be assigned in any kind of repeating pattern (i.e. every Friday, or the first Tuesday of the month), you can set this up by clicking the "Repeat assignment" button:
This will bring up the "Repeat assignment" dialog, where you can specify the pattern for the assignment, and optionally specify start and/or end dates as well:
The volunteer will fill the same position on this pattern in future schedules unless an Ending date is specified or the repeating assignment is removed.