Print out or show on phone for many services. Some companies/routes are only possible for interrail to send reservations by post, read the details of each reservation carefully.

Reservations and passes are not connected in any way. If your reservations have been sent as an attachment by e-mail then the advice is to print them out for inspection. 

Reservations have a wide variety of formats, some with their own QR codes, some with bar codes and some just a simple PDF with details on it.

In Italy and Eurostar you will need access to the reservation QR code as well as your Interrail QR code.

I haven't seen any reservations lately that need to be printed, except for night trains (were it is a good idea anyway).

Interrail always say that reservations they send via email need to be printed but that is mostly not correct. I guess they're just covering themselves in case some of the reservations do need to be printed.

The catch is that-but I understand only when most of the process has been done- SOME specific RES can only be issued by this system (and that differs from others) as paper and has to be sent by snail mail in an awful long time.

You get the general advice if you order via oebb.at (which is often mentioned here and the only viable way to do RES for TrenItalia beforehand), and as above: those for IT are better to print, though finding the pdf with the barcode etc to pass gates and show conductors should work-as such.

e reservations are invariably in the pdf format and take only a few seconds to print. Onboard train it means showing you QR code then swapping to a pdf reader to show your reservation. Far easier to me to have a printout and show it if needed.

It is also much more powerful when ejecting people from your reserved seat.

Thanks for all the replies