I've written a bit about reference designators here and there. There are a few more factors that we run into now and then. Take the family panel. In case you aren't familiar with the term, it means that you have several different designs laid out into in one panel, as opposed to multiple copies of the same design in one panel.
Using a family panel can be a convenient way to deal with a multi-board design and can sometimes save a bit of money. Just a caution, though. Make sure to check with your fab house first. Some don't like family panels and some won't separate them for you. If you do have them separated prior to assembly, either at the fab house or by you, then you don't have any reference designator worries.
If you leave them in the panel and wish to have them machine assembled, it can get a bit more complex though. "Why?", you say. I'll tell you why. Generally, most people start at "1" for each new design. i.e. "D1, D2, D3... R1, R2, R3..." If the boards go into the machine independently, that's no problem. However, if you send the panel into a smt assembly robot, it may very well see that as your board having multiple D1's, R1's, etc. That would be rejected as an error in most cases.
If you are using the family panel approach, don't restart your numbering when you move to another one of the designs that will be in the panel. Either continue on from the last number in the prior design, add in a hundred's, with each design getting a different hundred's number or add a unique suffix on each board.
- Wrong way: PCB1: "R1, R2, R3, R4, C1, C2". PCB2: "R1, R2, R3, R4, C1, C2".
- Right way: PCB1: "R1, R2, R3, R4, C1, C2". PCB2: "R5, R6, R7, R8, C3, C4".
- Right way: PCB1: "R101, R102, R103, R104, C101, C102". PCB2: "R201, R202, R203, R204, C201, C202".
- Right way: PCB1: "R1A, R2A, R3A, R4A, C1A, C2A". PCB2: "R1B, R2B, R3B, R4B, C1B, C2B".
There are a lot of ways to do this. Just make sure that no reference designators are repeated from one board design to the next. I prefer method #3 myself.
Is it immediate or extended? Does it matter?