Coverage works by companies and not types of trains. The vast majority of European trains are included. :)

All trains operated by BDZ (Bulgarian State Railways) are included. Train tickets in Bulgaria are very cheap so it isn’t worth to use a pass day.

All trains in Germany are covered except Flixtrain but it’s not really high-speed anyway.

Optional seat reservations in Germany, Austria and some other countries :

• 3€ per train through tickets.oebb.at
• 4.50€ (soon 4.90€) per journey through bahn.com for journeys to/from/within Germany

Only one night train between Budapest can be booked online, this one (through tickets.oebb.at) : 14:50 - 05:20. Add Interrail/Eurail as a discount and select one-way tickets. For other trains : book at train stations in Romania, Germany, Switzerland + some other countries if you happen to visit them before. Not sure about Bulgaria.

Hi thx for that information. If i need additional seat reservation can i just pay right on eurail app, also if i want to take another train time can i replace different train time in my journey

Reservations are 100% seperate and not connected to your Railpass. 
Some Reservations are refundable / exchangeable but not all and on some you pay a fee for the Refund/exchange 

On the pass itself you can change the trains at anytime but you have to be sure to have a reservation for the train if required