Estimate / Invoice Subject Line

Hello

I have a limited set of regular clients, not more than 5.

I raise at least 30 different estimates and invoices for them each month. I discovered SI a couple of days ago and have already started moving my lot onto it.

When I'm managing estimates and invoices I'm not ab;le to distinguish the projects they belong to. How can I add a subject line to my estimates/invoices? Can I use the custom fields function? Then how do I move it right on top?

Thank you