Student or Learner
I act as consultant for a foreign entity. I want to use terms as Jour J (D Day), mois M (M Mounth ?), année N (Y Year ?), niveau N-1 (level L-1 ?). Is there a rule or habits ?
Welcome to the forums.
So long as you make it absolutely clear to the reader/user what your abbreviations mean, there is no absolute rule. The best way is to insert the full word in brackets on your first use:
"In Y [year] 1, we will ....."