Yes, it is a somewhat confusing marketing terminology, but the short answer is that there is precious little available that you can add to HSSE in the line of synth sounds.
The full version of HALion 6 can be used to create “VST Sound Instrument Sets” which is a package of one or more “patches” with everything needed to play them, however there is currently only a very small emerging market for these. The bottom line is, HALion has a lot of catching up to do on the market leader, Native Instruments’ Kontakt. I think it will take time before people start creating additional sounds for HSSE, if ever.
In the shop at the moment, you’ve got likes of The Grand and HALion Symphonic Orchestra, which are confusingly listed under “VST Instruments” even though they are content sets which use HSSE as the player (i.e. the actual VSTi), and even more confusingly then there are the relatively new “HALion Instruments” listed here.
Maybe someone from Steinberg will come along to explain why this is all so confusing – it’s not due to “neutron” fatigue on your part!