SA100 TR 8 Box 21: How do you populate box 21 to show you are enclosing separate supplementary pages?

SA100 TR 8 Box 21: How do you populate box 21 to show you are enclosing separate supplementary pages?

Box 21 gets populated automatically if you add any supplementary pages to your tax return.

These pages include the Employment Page (SA102), Self Employment Page (SA103) and Residency Page (SA109), for example.