Plastic bottle codes

[sarah_cookie]

What do the numbers and initals found on the back of plastic bottles mean?
I've tried to work out whether it's to do with the bottles size or thickness of the plastic, but can't seem to find the connection.
They're found on the back of shampoo, handcream, soap, toothpaste, everything made of plastic, and has a little picture of a container without the lid and 12 or 9 or 24 M next to it.
Does anyone know what they mean??

[Supernick]

Here's the resin identification coding (not the ones you're after), but at least it's in context with the question

http://en.wikipedia.org/wiki/Resin_identificat ion_code

[4getmenot]

Means how many months you can keep it for. Which is really stupid if you ask me, if I looked in my bathroom cabinet I dont know when I bought half the stuff.

[suzy64]

It's how long they last before you should throw them away

[SteveD]

Aren't they something to do with the shelf-life of the product once opened (9, 12 or 24 months)?

[SteveD]

Yes, here you are. Open the PDF file and scroll down:

http://www.colipa.com/site/download.cfm?SAVE=2 1099&LG=1