(Do you have any of those quite attractive 'used' stamps from Outer Mongolia, issued in the the 1950s to 1980s? And how many letters have you received from Outer Mongolia with those stamps on them?)
And to pile confusion on confusion, there's a sort of subdivision of CTO: favour cancellations. These are not outright CTOs, churned out by greedy postal administrations, but stamps cancelled by an obliging post office employee, specifically to provide used examples.
If you lived in East Germany or Czechoslovakia for instance, it would probably be quite easy to clip Mongolia stamps off mail.
Just as in the Western world, "study abroad" means going to the USA, UK, Australia et al, for Communist countries, it was an achievement if the government gave you a scholarship to study in Ukraine, Bulgaria, Hungary. Usually something "practical" like engineering. The crown prince of Cambodia in the 1970s was a student in Prague.
When you become a more advanced collector and start perusing auction catalogues for higher-valued items, the term per favour
might come up. As you can guess, that means favour cancel--asking the clerk at the PO to cancel a mint stamp for you without actually mailing anything with it. Also known as "hand-back service".
I remember an auction here a year or two ago, that had a collection of early-ish (Geo V) HK stamps with rare cancels. A number of them were described as having been backdated, but for some of the small branch POs these were the only existing strikes of those hammers known. Someone with an inside connection in the PO had, in the 1940s, got hold of the old hammers and had some 'reference strikes' made on mint stamps so their collection would be a step closer to completion.