What 'Product Tax Code' Do you use?

Our products should have tax and my merchant invoices show the price inc VAT, yet a few FBA invoices show 0 VAT and i dont know why, i go into our product listings and they are all defaulted to GEN_NOTAX which is the only drop down in the flat file and we have always used that. However now some of our products have gone under A_FOOD_… i do not know why?? And how has this affected our VAT bill etc? We have software to calculate all the VAT etc and dont know if it would pick this up? Should we have been using a different code?