The Purchase VAT is accrued VAT logged against purchase invoices that has yet to be reconciled against your VAT return. You should have something similar for Sales VAT.
I assume you're referring to these accounts:
2200 - Sales Tax Control Account
2201 - Purchase Tax Control Account
When you file a VAT return it takes all those sales and purchase invoices in your VAT return, NETs the figure for the 2200 and 2201 account and moves it to the VAT liability code (2202).