"excuse" and "PES" are two separate things.
"excuse" meant that you are excused from certain activities.
e.g. you are excused from carrying a jerry can filled with water because of your "excuse heavy load".
"PES" determines that the locations on the battlefield where you can be deployed.
for instance PES A meant that you can deployed anywhere on the battlefield so you can deployed on the front lines, the support lines, as well as the field HQ.
PES E might mean that you can only deployed in the field HQ and not the front lines or the support lines.
therefore you can be a PES E storeman and still need to move heavy stores.