ON the G/L account card, If there is any direct expense account, example Electricity , Telephone , fees, etc where GST /VAT is required to be calculated on the amount of the expense, The General posting type is required to be "Purchase" and the general prod posting group is required as a part of setup to call the GST/VAT prod posting group.
Similarly Revenue accounts need General posting type as "Sale"
These fields help in reporting specially GST statement reports.
A combination of Gen. Bus posting group and Gen. prod posting group is required on the general posting setup.