BAS HVAC refers to the systems necessary for connecting your HVAC system to your wider building automation system (BAS). Specific BAS HVAC components include, among others, VAV boxes/terminals, a central air handling unit, a main workstation, and a server.
The main benefit of a BAS HVAC system is that it makes operating an HVAC system easier by digitizing most of its controls and more affordable.
The affordability comes from being able to operate the HVAC system more efficiently, such as timing it to reduce its cooling or heating when there are fewer occupants indoors, instead of cooling/heating at a constant rate 24/7.