This category will have a balance in it, if all of the VAT on your purchases is greater than the VAT on your sales for your current VAT period. When you lodge your return and HMRC pays you for the amount outstanding, this will net off to zero for the period lodged.
What's included
When the VAT on all of your purchases outweighs the VAT on your sales.
What's not included
Any of the other taxes such as corporation tax, PAYE or NI contributions.
You should only categorise to this category if you are receiving a VAT refund