To create order rules, you need the Order Rules module.
The PopulateVATNumber order rule action means that if an order doesn't contain a VAT number or you don't want to use the client's VAT number on orders then you can populate one for them. For the Param you would enter the VAT number you want to add.
The PopulateEORINumber order rule action does the same as above but instead of populating the VAT field, it will populate the EORI field on the order. For the Param you would enter the EORI number you want to add.