At the start of a pz2 file it says:
version
    {
    number 7
    }

---

Older pz2 files says "number 5".
If a product is to work with P5 and P6 do I need to change the version number (to 5) or can I leave it at 7?

Generally the older versions will still load the file but a rather annoying error message will popup telling you that Poser doesn't recognize the version number but will attempt to load the file anyway. 

As I said, generally it will load, but if you're going to offer the file to others it's a bit more considerate to change the version number just to spare potential users the annoyance of an error message.  And possibly spare yourself an onslought of irrate e-mails from annoyed customers complaining about it..

Hey! His nose is dry! ... Someone should lick it,  just in case. - Diego

Thank you SYNTRIFID, good advice. I will change it to a "5".
**
**

I generally change the version number to 4.03 so it will load without the error message no matter what current version of Poser is being used. Not everyone uses 5,6 or 7 As SYNTRIFID said, it is just a consideration, not mandatory

I must remember to remember what it was I had to remember.

Good point nickedshield, but this product will only work with versions that handles dynamic hair. If I remember right it starts with P5. Or was it P4?
hmm... now I'm confused. Anyone remember?

P5 was the first Poser with dynamic hair.

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"you are terrifying
and strange and beautiful
something not everyone knows how to love." - Warsan Shire

Thanks karen1573. Glad I'm not all senil yet....