There is no single “correct” answer here, so it’s up to transcriber discretion. In the end, so long as the language is in the heading, whatever wording you use will probably be fine (otherwise you will have to define the individual symbols as “Spanish a with acute symbol” “Spanish e with acute symbol” etc.)
My thinking about this is colored by the fact that when we do the workshops and webinars we’re trying to make recommendations that can apply to multiple languages. For Spanish, the only braille symbols we will use are accented letters, so it would be accurate to label these symbols as “Spanish accented letters” or “Spanish alphabetic symbols”. In other languages, there are symbols that don’t fit as easily into these categories (like a glottal stop, or the Ancient Greek breathing marks), so in some cases a broader heading is necessary.
When I have more than one language, I put a heading on each set of symbols. UEB symbols are “Unified English Braille symbols”, so for consistency, it would make sense to call the other symbols according to the code they are taken from. If we wanted to use the official name of the code, the problems are that 1) the name can be difficult to find, and 2) the name is probably in that non-English language, and then we have a question of how we are brailling it. So that leads me to prefer a descriptive heading like “Spanish braille symbols”. “Spanish braille code symbols” would be about the same. What I want to communicate with the heading is that these are symbols taken from Spanish braille – I’m not sure that “Spanish accented letters” says the same thing, because we could have a UEB representation of a Spanish accented letter within the same book (e.g. in an accented name within English context).
In terms of “terminology,” I think you mean that we are not using that code’s definition of the symbol? I don’t think there’s any way of avoiding that – we are in a UEB context and need to define print symbols in a way that makes sense to a reader of UEB. (I could write several paragraphs on what I think about defining non-English symbols but I won’t unless you really want that.) In many cases there is more than one reasonable way to define a non-English letter.
One final note: I think it can help to put a note on the Transcriber’s Notes page about the transcription; this can help add context for the special symbols. For example, the TN page could include “Spanish text is transcribed uncontracted, using Spanish braille symbols for alphabetic symbols and using UEB for punctuation, indicators, and other symbols. Spanish braille symbols are taken from World Braille Usage, Third Edition.” Adding this information about where the symbols come from could maybe take some pressure off the symbols heading?
Let me know if this wasn’t helpful enough, or if this brings up more questions/issues.