They created 'pools' of HXP and HSP for people who have more than one copy of a single variant (eg. 3 HBK-SPs). That way you can decide how much you want to transfer to each individual mech.
HSP is the Historical SP each mech received for the skills it already had unlocked. If your mech was Mastered it received 91 HSP to unlock a full set of skill nodes.
When you click the Add SP button you get a choice of using HSP, GSP, or HXP/XP to add SP to that mech.
(Just a tip, if you don't know how the SP/HSP/GSP/refund system works you probably shouldn't be answering people's questions on them.)
Edited by MadBadger, 16 May 2017 - 02:42 PM.