Brit Mil Acc & Abbrev

After a long wait, several months, I finally received my mother's WW2 military service file. She served with the ATS (Auxiliary Territorial Service) in England. This extract shows promotions while serving in the ranks. The military was and are, renowned for using acronyms and abbreviations. Perhaps you can guess what they mean.
Helpfully, the covering letter referenced a web page that links to a 16-page pdf of Useful Abbreviations, from  A for Acting or Assistant to ZION MULE C. Zion Mule Corps2. It did help a bit; wef is "with effect from." Some are not included like P, U and W.
Does S stand for Staff? It seems probable as she'd been a corporate secretary before enlisting.