I have not seen it yet.

'Yet' is most often used with the present perfect in negative and interrogative sentences to refer to an action that we know will take place if it hasn't happened up to now or so far. 'I haven't seen that film yet' (but I am planning to see it) 'Yet' can be used in the same way to imply the negative of a following infintive. 'I have yet to see it' but this structure isn't so common and we normally use the present perfect instead. But they mean the same thing. I have yet to see it = I haven't seen it yet.

Or it could mean that you are sceptical of something occurring or taking place.

Using "yet" means that something hasn't taken place, but is expected to take place in the future. The above two answers sum up the two most popular uses.