But "affix" can mean not only "attach physically" but "attch in any way: add, append". <affix a signature to a document> (According to Merriam-Webster)When I sign a contract that must bear an official corporate emblem, I make an impression on the paper with a metal device; this impression is called the corporate seal. (It's not something that's affixed.)
Your guess is based on a document in Nigerian English and refers to the verb "to seal" rather than the noun.But you both still didn't mention "stamp". If you looked at the following link, you would see "stamp" is used instead of "seal". So I guess "seal" and "stamp" can be interchangeablly used.