are there set splits for a run? eg do i have to time by bosses, or levels. not sure if this applies to different games but theres nothing that says what your splits have to be

You can have splits wherever you want. The only times it matters is when you start and end the timer you can have 0 extra splits or 100+ it doesn't matter the game or anything but most communities typically split at the same points.

I agree with what Tadyman said. The only important things are starting and stopping the timer, everything in between is all up to you. But it is nice to be competitive with the community. Often you can find a set of splits by going to the leaderboards, click "resources" on the left and scroll down to splits. Otherwise, it's all up to you!

In case you didn't know this yet; splits are purely for analytical purposes. You can find out how fast you do a certain segment (think of a level, time between 2 bosses or between 2 items) etc. So that later, you know what wasn't really good and might need some practice. Or whenever you know you really got a best time in some point.

Splits themselves are redundant (optional better said) but sometimes showing the timer itself is mandatory due to timing methods such as loadless real time or IGT which use the timer to read memory from the game. In any case even then a LiveSplit timer is never really mandatory. Its just that when you choose to run in RTA instead of loadless realtime or even instead of IGT, one will typically end up with a slower time anyway.