Verification is a process to check if the input is equal to the output and every input has to have an output.
Verification phase has to be in the execution phase since by that time most of the basics are covered such as the DID and DSD and any change will have to be approved unless it changes the overall purpose of the device we are manufacturing. There is no rigid rule that it should be done in the execution phase itself but since most things are sorted out in the execution phase, and before going to the next stage we need to verify that whatever work has been done is correct as per our requirement verification is done.