"I think the aim is to have the right FT and PT mix in the office everyday. eg if there are 15 walks but you only need 10 FT and 5 PT to cover the indoor then you will have 10 FT walks and 5 PT. Assuming a 1 in 6 rotate you have 2 FT covers and 1 PT. The duties are picked in seniority order whether or not FT or PT. When 10 FT walks are filled then the remaining FT can only pick the cover that will be created from the 10 duties picked or be reserves. The same would apply if 5 walks were filled by PT first (less likely). So there will be 10 FT and 5 PT walks but which ones will be decided by the seniority pick.
Hope this is right and helps.