SEAT RESERVATIONS

Reservations generally cost money, with a few exceptions, like the UK.

Use this guide to check prices and where to book:

https://www.seat61.com/interrail-and-eurail-reservations.htm

Edit: I now see you booked the Plus pass. This means most reservations are included, as long as you book via Interrail. Only the most expensive reservations require a "credit”, of which you get only 3. See this page to see which reservations are free and which ones use a credit:

https://plus.interrail.com/en/global-plus-pass

(Click on "List of included countries and railway carriers”.)

Eurostar needs a credit, so you'll only have 1 left after booking a Eurostar return.

Thanks for the prompt reply. I’m still not sure. In the plus pass detail it says “secure most seats at no extra cost”. Book most seat reservations through our timetable at no extra cost and enjoy up to 3 extra credits to cover the reservation fees of some popular trains (1 or 2 credits per seat).

How many credits are available before you need to use the 3 extra?

I have not purchased the pass yet.

The ‘credits’ system is only for the more expensive trains - Eurostar, TGV etc. 

All other reservations, including those on German or Italian domestic high speed trains, are effectively unlimited, and don’t use any of your credits  

As ​@rvdborgt points out, those of us coming from the UK will use 2 credits on our Eurostar journeys, meaning only 1 is available for TGVs etc. Beyond that, you’ll have to pay for your TGV reservations.

I don’t think the Plus Pass is a great product, to be honest. It might save you some money  - particularly if you’ll make lots of reservations in Italy - but otherwise it’s a bit of a faff.

Thank you again. You’ve given me a lot of good info.