@chartsqa, if you mean referral links, then they are prohibited in posts (except in some exceptions), but you can put such links in your signature as well as in the Website URL and as Personal Text, which you can find by clicking on Profile -> Forum Profile Information.
There are some gambling threads here in BitcoinTalk that use invite codes to track their engagement here in BitcoinTalk. I think this is only allowed if you are the representative of that casino. However, I haven't read about this; I've seen many gambling threads doing this even in signature campaign threads with image hyperlinks.