The "(sub)" label will only appear after a volunteer's name if "Put '(sub)' after the name of substitute volunteers" is selected, and the volunteer is a substitute in the ministry that is currently being listed. That is, "Group volunteers from each ministry into separate lists" must be selected, "Put '(sub)' after the name of substitute volunteers" must be selected, and the roster must be listing a ministry in which the volunteer is a substitute in order for the "(sub)" label to appear.
Note that the "(inactive)" label will appear after a volunteer's name regardless of where the entry appears in the roster if the volunteer is inactive (and the appropriate option is turned on in the "Name modifiers" section for the roster).