Based on @Glenn’s post here:
http://community.quickfile.co.uk/t/purchase-reference-for-recurring-invoices/2189/5?u=parker1090
If you want the reference to carry forward to new purchases (for example, it may be a part invoice for a bigger job), then prefix it with “PR:”
However, if it’s a one off reference (for example if may be rent invoice every month with a different reference), then don’t prefix it with “PR:”, just enter it s as it is