Picture of corner trim as installed.
Gable end siding shiplap.
The corner trim gives a solid edge to fix the siding up to and it also gives some end protection to the end grain.
Instead of simply running the siding material all the way up we often use shiplap boards to set off the gables.
Then add 20 on the gable to cover the waste of all those angle cuts 96 x 20 115 sq.
A three person team two installers working from staging who feed measurements to another worker at a cut station on the ground can side a large gable end in an hour or two.
I started installing the shiplap cladding on the gable panel.
Measure between the trims at the top bottom and middle this will tell you if the ends of your shed side panel are square.