As I understand it, the usage in the RCN was to distinguish, during WWII, between officers of the permanent force navy and those enrolled for the duration of hostilities only in the temporary force. Most RCN officers enrolled during the war had "Temporary" rank; they could also have "Acting" rank if they were in a position higher than their established "Temporary" rank. Permanent force officers could also have "Acting" rank.
There was also something called "Probationary" rank, leading to the wonderful "Probationary Temporary Acting Sub-Lieutenant (P/T/A/Slt)", but I have no idea what that could specify, and I'm not certain how all the various prefixes align with deemed seniority, and with the fact that most (but for whatever reason, not all) of the temporary force officers were also considered to be RCNVR (reserve).
It's all a rich tapestry.