A gable roof has two opposite slopes that meet at the ridge while a shed roof slopes in one direction only. Easy Rafters is designed to calculate shed rafters that are framed to a ledger board nailed against a vertical wall of a building, not for free standing shed roofs supported by walls at both ends. The results obtained with Easy Rafters can be adapted for that type of situation by manually adding a second birdsmouth cut with the desired overhang at the peak.
The distinction between gable and shed is necessary for Easy Rafters to calculate the proper thickness allowance for the ridge or ledger board. Easy Rafters subtracts one half the thickness of the ridge board from the theoretical run of a gable roof common rafter or the full thickness of the ledger board from the theoretical span of a shed roof common rafter.