Invoice titles?

Invoice titles?

In my old system we had titles for our invoices and this makes it easier to identify the invoice I am looking for in a long list. Zoho doesn't seem to have this though and it is a hassle every time I want to look for a particular invoice. Is there an option somewhere that I have missed or that I can turn on where I can give the invoices titles?
I have also found the tour not to be very helpful.

Could you also please tell me what the PO# is?

Thanks.