There are trains where reservations are not possible (most trains), others where they're optional and some where it is mandatory (often high speed).

Reservation prices vary a lot, from free of charge (UK, Greece) until €68 for a seat (Paris-Basel/Zurich 1st class). Couchettes and sleepers always come at an extra cost.

You can find a list of prices here:

https://www.interrail.eu/en/plan-your-trip/reservations/reservation-fees

Your pass is your travel ticket for all trains operated by the participating companies and this is the vast majority of trains across Europe.

https://www.interrail.eu/en/plan-your-trip/tips-and-tricks/trains-europe/railway-companies

The only extras are reservations and a few supplements for a few trains from Italy across the Alps. Sleeper supplements/reservations can seem expensive as you only save the basic fare but still have to pay for the cabin/couchette.

Just avoid the few private operators such as Italo and Ouigo.

Ouigo is just SNCF under another name...

But you cannot travel on Ouigo trains with a pass - unless it has changed recently. You can travel on Inoui TGVs and all SNCF regional and local trains with a pass.

That's correct but there's nothing "private” about Ouigo. It's just SNCF who have decided not to accept passes on these trains.

Many apologies

I thought you were suggesting Ouigo were included in the pass. Many of these "private" or "not included" companies are indeed fully or partly owned by national operators. But that is a debate for other forums.