Poland EC reservation

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I know that reservation isn't obligatory if passenger travels on international route with EC, and that it is obligatory if passenger travels on domestic route EC. But I'm not sure about traveling domestic route with international ticket. For example, passenger buys ticket Zilina - Warszawa via Skalite(Gr), and changes trains in Zwardon and Katowice. If passenger chooses to board EC train between Katowice and Warszawa, is the reservation obligatory for that EC train?

I know that reservation isn't obligatory if passenger travels on international route with EC,

In Poland?
Are you sure?

The order you write about - applies in some countries, for example in Hungary.
But in Poland - reservation in EC is obligatory even in international connection, AFAIK

Well, first of all there is probably a need to distinguish two things with higher rank trains (like EC) in domestic travels. Time to time they get a bit mixed up.
First one is supplement/special tariff. With PKP it is even separate tariff table (obviously dependable on distance), while with companies like MAV it's just a fixed fare supplement. And you definitely avoid this (let us call it "EC surcharge") when using SCIC-NRT ticket on domestic route (like with Zilina - Warszawa boarding EC on segment Katowice-Warszawa), but we all understand that this is not the issue here. Second thing is actual seat reservation itself, which is in your case unavoidable, since any EC trains from Katowice to Warszawa are those coming from Czech Republic (111, 104, 102) and requiring reservation, as already noted by Maxy, on segment Bohumin --> Warszawa Wschodnia. Both CD and PKP websites state something like "povinná rezervace míst" here, thus in one of those EC trains after Katowice conductors will definitely be interested why you don't have a reservation, which is required both for international and domestic travel.

Thanks for the answers