Good question. “Commutable” refers to the ability to get from wherever you live, to your base, or vice-versa on the same day as your trip. “Non commutable” means having to travel on a day off.
For example, a trip that starts at 8 am would n to be “front end commutable” because clearly you would have to come in the night before for that trip. Now one that starts at 4 pm is likely “commutable” because you can probably fly into your base that same day. Same logic applies to the back end of a trip.