The wiki starts with:
osmc:symbol=* is machine readable equivalent of symbol=*, both describe route symbol that is used as waymarker or on guideposts
I think that is clear enough?
BTW many route markers include the ref. A short code or a number. The routes I operate have stickers and small shields with the symbol; I use them about 66.6% without and about 33.3% with characters on them. If there is a ref number or code I usually include that in osmc:symbol. The name also appears on many stickers, especially where routes cross each other, I put that identifier only in the name tag.
BTW2 I once suggested in the Dutch community to replace the made-up reference codes (e.g. Pi for Pieterpad, FV for the Floris 5th trail) with their official route number issued by the national operator. This idea did not fall into good earth, as we say. So we are stuck with the mnemonics, for now.