What does /M meaning in a hunspell dictionary

I added a word "isoceles" to the hunspell dictionary and it worked OK. I noticed that many entries have a flag /M (and there are other flags), but I couldn't find documentation of the flags anywhere. Are these flags something I need to know just to add a handful of words?

I think it is arbitrary (not that it is like "male gender" or something) and the meaning is given in affix file next to the .dic file.

For example the open office one or some github one have the same meaning -> the word can have 's. So the library can generate other possible forms of words.. For son it can generate son's and so on.