A VAT rule group is the definition on how to calculate the VAT for an administration.
In europe, the VAT is usually based on the AOC, but in the UK for example FBO One can charge VAT based on the UK rules (country of operator and MTOW of aircraft)
FBO Locations that share the same VAT rule group calculate the VAT the same way, but also share the VAT related fields in contacts (AOC/VAT Remark, Validate AOC and Charge VAT), while locations in different VAT rule groups can have different settings (and defaults) for these fields.
VAT rule groups (Administration)
VAT rule group (Fields)
VAT in Europe