Article 22, Section 4 says you have seniority rights to your job category (like loader or unloader or sorter, etc.) but usually not to any specific position within that job category. So if they are looking to take a Small Sorter, say, and make him load, they should first look for a Loader, and if there are none, take the lowest seniority Small Sorter.
See also the Seniority Article in your Supplement.