Administrators and captains can create a generic substitute request on behalf of the adoration program, so the Substitute Finder can look for someone to cover a time nobody has committed to. And when additional commitment start times and durations are enabled, adorers can commit to anywhere from 15 minutes to 2 hours.
Now those two work together. When you request a substitute On behalf of the adoration program, you can choose a Start time and a Duration. If only half an hour needs covering, your substitutes are asked for exactly that half hour.
These choices appear when the Enable additional commitment start times and durations field is turned on at Admin » Settings » Commitments.