The abbreviation TIN normally stands for Tax Identification Number. Each country has its own term for it but TIN is used pretty much worldwide. In Greece it would be ΑΦΜ and it is on the front of the tax bill one up from the bottom of the group of items that appear under the ΔΕΗ logo. The heading is ΑΦΜ/ΑΔΤ. It doesn't have an explanation next to it on Bob's link.