[foo_barrio]

Is it used only for counting? Mandarin and other East Asian language have a classifier for counting. English uses counting classifier sometimes. For example, "paper" without a classifier in English refers to an official document or essay. For example "I have to write a paper" or "Do you have your papers?" otherwise you have to use "a sheet of paper", "some paper", "a pack of paper" etc. It's not exactly the same as the counting words in Mandarin but play a similar role in grammer.

[packet_nerd]

Yes, sort of, but a lot more than counting. I think Thai has counting words too, but I think they are not as central to the grammar or as flexible as Karen classifiers.

Sort of like you can say "a sheet of paper" or "5 sheets of paper" in English to count papers, but imagine you could also say "typing on a sheet", or "your sheet is full of typos", or "could you hand me a sheet", where "sheet" is a broad category and that you mean a sheet of paper comes from the context in which the sentence is spoken.

Edit: Another interesting use for them is disambiguation. Super useful if, like me, you're just learning and don't always nail the tones or pronunciation. For example, I might throw in the animal classifier in "ga cha ta doo" just to make sure no-one misunderstands my poor pronunciation of "elephant" as "mountain". That's a crude example, but native speakers benefit from the disambiguation too in colloquial speech.

[bradrn]

Words with this range of meaning are simply called ‘noun classifiers’ in linguistics [0]. They’re pretty common both in the region and outside it: they’re also in Lao, Hmong and Minangkabau, as well as various Australian, Mayan and South American languages.

[0] Aikhenvald 2003, chapeter 3: https://global.oup.com/academic/product/classifiers-97801992...