I think people (even some that work for SYMC on EV) have had some confusion about this for some time. Just to set it straight I will add an area I think confuses people. When you set an indefinate retention alone... you can still delete the item. If your retention has passed... but the ret cat prohobits items from being deleted .. it will not be.
Legal holds, legal interests, compliance hardware, archive permissins, and a site setting can all prevent deletions as well.
Expiry on the other hand is the setting used in conjunction with the retention period and the storageexpiry process. THis permits you to age out items that are no longer required. It complements the idea of data management and that keeping stuff forever could put an organization at risk ... because they are liable for whatever data they have . Defensible deletions are a big deal these days.
I know I got a little off topic... but I cannot tell you how many people I have seen lower retention ages in an effort to delete items off NTFS... who should have known better.
I hope this heps and you recall to note whoever helped you here and close down the topic when you get all you need from it.
Regards.
j