It does vary by series - at the Britsh XC you need a licence for anything but Fun/Open if you're doing more than one round (you certainly did, they may have changed it).
However it's advertised, sport will always be an option - BC will give you a sport licence even if you're technically a master (just as plenty of the elites are actually masters, and even vets in a few cases).
Open is just that - open. It always used to be called novice, but folk wanted it changed, then moan it's full of 'pot hunters'. No pleasing some.
Sport will be easier than masters at the front end, but generally a bit more consistent, masters can drop off further back in the field.
If the time and the duration suits you, just race open!