"Is there some kind if MN rule that facts can only be included if they are relevant to the specific issue?"
When a community has an established jargon, then yes it is the 'rule' (or rather the norm) to follow it.
Mother = the actual female parent with whom the child resides (whether by birth or adoption, and including when residence is shared)
Bio-mum = if another person is in role of mother, this refers to the female who gave birth, and who may or may not still see her DC (depending on whether it is fostering, SGO, open adoption, or not).
Step-mother = wife/partner of the children's other parent, usually only after cohabitation has begun
Yes, other sites do it differently (as noted above), that does not however alter the MN norm.
(Which it seems OP has had pointed out to her on successive threads, so it does beg the question why she chooses to continue to ensure her threads are de-railed and that some at least wonder about her self-awareness)