There is a project tagging feature which allows you to group purchase and sales invoices (as well as estimates, recurring invoices etc) to a project tag.
The only limitation I would say there would be, is that the whole invoice would be tagged to the invoice, and you can’t separate it line by line.
The system is definitely easy to use, and any problems, there’s an active community here (as you can probably see yourself) who are always more than happy to help 
Edit
For reference, here’s a nice little post on the project tags:
http://community.quickfile.co.uk/t/projects-in-quick-file/1591