Depends on the route/company but no, it's not unusual. You've got 5 seats listed, good to go.

Staff won't want to see optional reservations. You only make them for peace of mind and to eject potential squatters. ;)

There are usually small screens or paper slips above the seats too.

It was a seat reservation required.  I added the trip plans and added all 5 individual pass numbers to the trip and then selected second class seats.  Now i’m reading stuff about a Pass Cover Number?  Should I have done this?  I really hope that i’ve actually reserved seats for all 5 of us.  It’s so confusing.

What's the route/company please? If you want real advice you need to give more details.

If the place where you booked didn't ask for a pass number it's 100% fine.

BRUXELLES-MIDI (Belgium) to KOELN HBF (Germany) via seat reservations on the app

I'm assuming an ICE train (run by DB) ?

These trains have optional reservations most of the year, except in summer due to overcrowding. The booking system is still the same: only one name is required.

No need for a pass number here.

Here's a good guide about the best places to book: https://www.seat61.com/interrail-and-eurail-reservations.htm

Thank you for your advice.

There doesn’t need to be any confusion.  You booked 5 seat reservations, you got 5 seat reservations.

Whether the reservations have any name on the or not doesn’t change anything - you are now the holder of a reference for 5 seats on the train you wish to take.  The seats are reserved.

Unlike air travel there’s generally no need for all the masses of identification checks with European rail travel.  Buy ticket to travel (your interrail or eurail is yours), sit in a seat or reserve a seat.

All is good.  Enjoy your trip.