Hi @Woo, to be honest it’s mostly a case of semantics and marketing
We use XD as a broader catch-all term; partly because UX is very much viewed as an evolution of UI, or Interaction Design (IxD) with a greater scope and we wanted the event to be inclusive of a greater number of disciplines of User-Centred Design with that digital focus. So yeah - HCI is very firmly within that realm.
The design capability of my employer is often referred to as XD, but within that we have UX/Interaction designers, content designers, user researchers, service designers, visual designers, design strategists…
The other reasons for going with XD is that ‘Birmingham UX Group’ was already taken… and it meant that we could use BHX in the name