It would be nice if you can add the client-id to the Invoice Format, so that you can have something like

{clientid}-{num}
{clientid}-{num}{month}
{clientid}-{num}{day}{month}

maybe an individual nummering per client like:

{clientid}-1 (first invoice)
{clientid}-2 (second invoice)
{clientid}-3 (third invoice)

etc.

Would be nice to have. But just to have the option to use the client-id on invoice format would be a nice thing.