End Insertion

This setting is the percent probability that when a shape undergoes a Command Insertion Mutation, the new command is appened to the end of the shape's command list.

This setting is normalized to sum to 100% with

Middle Insertion

when you leave this dialog page, so it may not retain the same numeric value when you return, but its relative value will be equivalent. That means that to set this value as high as you can, you must set Middle Insertion to a low value.