What is the difference between the T20 and T21 tax codes? Used for both sales and purchases?

SOLVED

Hi,

I have been using the T21 Tax Code (CIS Reverse Charge - Standard Rate) for any sales invoices going out and any purchase invoices coming in wherever the Reverse charge applies.  Is this correct, do I use the same code for sales and purchase invoices?

Also, there is the T20 Tax Code (Domestic Reverse Charge) - What is this used for?

What is the difference between the T20 and T21, is there circumstances for when one should be used and when the other should be used?

Any help would be appreciated, I have googled and googled and can't quite find a straight answer! 

Thanks

G