It is normally standard practice for travel agents (and airlines) to only charge the 700 THB TS tax if your transit time is more than 24 hours. But if you tell your travel agent (airline) that you'll be exiting the airport during your transit, then I've found they'll normally include it even if the transit time is less than 24 hours. If you will stay in the transit area, you should check that BMI is not charging you for the TS. If they are, tell them you won't be leaving the transit area, so please delete the TS charge. If you don't pay the TS tax but then decide to exit the transit area, you'll have to pay it in cash, in Thai Baht. But if you're charged the TS tax, I think there's no way to recover it if you don't exit the transit area. Thus if you're ever not sure, IMHO it's best to have it ticketed without the TS, provided your company isn't paying for your travel.