A service charge is a percentage added to a bill by the venue, commonly 10%. It is a pricing decision, not a tax, and it is usually applied to the subtotal before tax is calculated.
Because it is not a tax, a service charge is entirely at the venue's discretion, and practice varies by market. In Thailand it is common in hotels, hotel restaurants and higher-end dining rooms, and generally absent from casual restaurants and street-food shops.
A POS should treat service charge and tax as separate components, in the right order — otherwise the receipt is wrong and the reports conflate two different things.
Example
A 100 bill with a 10% service charge becomes 110, and tax is then calculated on the 110.
Free tools for this
MinimalPOS handles orders, kitchen, billing and reporting in one flow.