• M means unambiguously masculine
• F means unambiguously feminine
• N means unambigouusly neuter
• X means underspecified for gender (i.e. M, F, or N; so all genitive plurals would become GPX, except perhaps in some participles)
• Y means non-feminine (i.e. M or N)
• Z means non-neuter (i.e. M or F; animate)

I do not believe there is any case where "F or N" contrasts with M.