In the UK, this is not a standard expression, though we would probably guess that it was meant for an official purpose. I found this : "A letter of accommodation can refer to the letter written by a person with a disability requesting reasonable accommodation and support for their situation, or the letter written by a property owner confirming that a Visa applicant has verifiable accommodation upon their arrival in the country." http://howtowritealetter.net/letter-of-accommodation.html There are two official uses for this term in US government administration. I can well imagine the term has other uses too, in various English-speaking countries.

I have never heard this term used other than the ways Michael Chambers has said above .However "letter of recommendation" is common.