exactly at the border (Astara town, between Azerbaijan and Iran, near Caspian sea). when agent didn't allow me to write exit point "Anzali" in Carnet, I went directly to the border's chief and asked what the hell is going on.. they said me that this is the rule, that "if you enter to Iran with Carnet then you must exit to the same border point where you entered. If you want to exit to another country, then it means that Iran will be transit country for you, and you need to have transit documents."
As I understood from what they said later: when you do transit documents, some iranian transportation company must "control" your vehicle. they write that your vehicle is under control of "ABC company", give all certificates, transit route table and other stuff and then you can go through Iran. There are several transport companies for your choice and from every company there is an agent at any border point around Iran.
I can post that documents, if someone can read Farsi.
All this sounds too hard and fantastic, and I didn't believe them even I entered Iran, I thought that they fooled me and just took money for nothing... But maybe such customs procedure is true, because Iran like paper works, and for every move you will need a bunch of papers. (for example, when I exchanged 100 USD to Iran Reals at the bank, I posted about 5 signs, then stamped 4 papers, then they made copies and only then exchanged me that $100)..