Okay, so they're all 'real' in the widest sense of the word. But there are different types with different etiquettes and anyone wanting to attend a session really wants to know which type they are likely to come across in any given venue.

At one end - a free and open session
Then an organised performance of booked performers but other known musicians might be accepted
and at the other end - a performance (that might resemble a more informal session but isn't, often characterised by half a dozen accomplished musicians who regularly play together sitting facing inwards in a tight circle and really playing to each other.)

I've seen all 3 of these and other combinations together in the same town both in England and in Ireland.

Another factor is why are they there, to play for pure enjoyment, to play to an audience or both. Personally I prefer it when it's both.