It's the non-scoping reference. Positional arguments are all required, so for instance in English you could say either "I ate" or "I ate a burger". But in my conlang, if a word is defined with 3 arguments, then 3 arguments must be provided. "ny" is kind of the escape hatch for when you want to not provide an argument but the grammar requires you to. E.g. "I ate" would require you to say "ate me ny" (verb first followed by it's arguments). So if you think about filling out an expression, the audience should evaluate it like:

"ate" (ok. Somebody ate something)
"me" (ok. The eating was done by the speaker)
"ny" (ok. No value was provided so I'm left with exactly the understanding as if I had only been given the one first argument. But also, sentence complete)

It's like, filling out an expression narrows into more and more precise meaning. But "ny" gives you a way to say "I gotta provide a value here. But I don't want the scope of meaning to narrow or expand or change in shape" but while still being grammatically correct.