CurrencyCategory::ParentCategoryID is a guess based on the fact it would make sense for all old expansion currencies to move under "Legacy," but I haven't confirmed this new organization in the client yet.
CurrencyTypes::WarbondTransferPercentage is based on Blizzard stating that some currencies will have a "Transfer" button in the UI to send them to another character and that some currencies will be penalized during transfer (https://worldofwarcraft.blizzard.com/en-us/news/24061008/the-war-within-warbands-preview). This also needs to be confirmed still.
SpellMisc::PvPDurationIndex matches the known PvP duration for every spell I checked. Some examples from the Mage class are Polymorph (118) at 6 seconds, Slow (31589) at 8 seconds, and Dragon's Breath (31661) at 3 seconds.