When you have an open ticket from Czech Railways (ČD / České dráhy), you can take any train category: Os, Sp, R, Rx, Ex, IC, EC, railjet/rj. For SC, you need a compulsory reservation.
Be aware of “SuperAkční jízdenka” which is a super-saver fare valid for selected train only.
There are also various other railway companies in Czechia like ARRIVA vlaky, GWTR, JHMD, KŽC, LEO Express, RegioJet, Trilex, Vogtlandbahn where ČD tikets are not valid unless noted explicitely.
Credit:stackoverflow.com‘