It's referring to its salt form, when you have a compound in its basic form (or freebase) it is often a pain in the ass to work with, most amphetamine and similar (presumedly, also benzofurans like 6-APB) exist as a freebase in a sticky, annoying oil form only. To get it to be a crystal, you need to "salt it out", which involves adding something to acidify it, you'll often see things in HCl (Hydrochloride salt form) or HBr (Hydrobromide salt form) as the results of using hydrochloric or hydrobromic acid to salt them out. If succinic acid is used, you get the succinate.
These are all relevant because they alter how much you should dose, succinic acid weighs more than HCl, so each mg of 6-APB succinate is a higher percentage succinic acid than 6-APB, relative to each mg of 6-APB HCl, which is more 6-APB relative to the HCl in the crystal.
This all assumes that it was salted out perfectly, which almost never happens though. There are many confounding factors in chemistry at large.