$clientCounter VS $clientIdNumber - can someone explain?

What exactly is the difference and where could I read up on these variables?

The client id number is the client’s reference number, the counter is used to determine the next id number.

Note: this feature needs to be enabled on Settings > Invoice Settings.

Thanks, I get it. I think if I use $clientIdNumber in the invoice numbering scheme it’ll reflect the actual clientID while using clientCounter would simply reflect the next clientID which would be assigned to t he next client I create. (If I got this right)

That’s correct