Why are there 3 nominal codes for VAT?

SOLVED

Hi all

My system shows the following 3 nominal codes for VAT:

2200 Sales Tax Control Account

2201 Purchase Tax Control Account

2202 VAT Liability

Surely, VAT liabiity = Sales Tax - Purchase Tax, so why have a seperate code for VAT Liability?

Secondly, code 2202 shows a sum which is not equal to Sales Tax - Purchase Tax.  So where has this number come from?

Any help greatly appreciated!