There are connections that do need a seat reservation (the ones that include an Eurostar train), there are connections that don't need a seat reservation. Depends on which one you do choose (you didn't tell so we can't know). The rail planner should show it when a reservation is mandatory. 

Personal recommendation: I'd avoid Eurostar (ES); very expensive reservation fee for a trip that is just a little bit faster. Do use IC trains (without reservations).

Of course you do. That's obviously as you don't have a reserved seat. 

Personal recommendation: No need to worry. 

Thank you for your reply. I am happy to travel on the slower regional train indicating No seat reservation is required. From what you say, I gather that even with a first class Eurail pass, I risk having to stand if I do not purchase a seat reservation even though the timetable advises no seat reservation required. Not at all obvious to me I am afraid. Perhaps senility is visiting me early!

The slower train is no regional train. It's a long distance Intercity train, too. 

You can't do a seat reservation for IC trains on that route. It's simply not possible. But that does mean there are no other travellers that hold one, too.

For other trains / connections: Not required does mean that you are allowed to board the train without a seat reservation. If you'd like to have one you should do one. But you don't need to reserve a seat; if you don't care you may save money and choose an empty seat when boarding the train.

But for that connection you did ask for it's not even possible to do a reservation so there's nothing to decide.

Thanks. The timetable said regional train, taking 3hrs 8 mins of journey time with a change of train at Anvers. So when the timetable says No seat reservation needed, it actually means you cannot buy one even if you wanted to with the objective of guaranteeing a seat for a longish journey with luggage? Is the only way to lock in a seat to choose a train with the mandatory requirement and buy a seat for that train (at considerable expense)?

Here the timetable does say Intercity train (IC) every hour (and that's true):

What connection with regional trains do you see?

No, that's different for every country and every train category. Very often a reservation is optional, possible and cheap.

But in Belgium and the Netherlands it's not possible to reserve IC trains. 

But you are worrying where there's nothing to worry. Most connections do run twice an hour. That's why there's no need to do a reservation in these countries. 

There's also no need to plan ahead that exactly. If you are ready in Gent, you head to the station and board the next available train. Rainy day? Leave earlier. Love the city? Take a later train.

If you are in Anvers and you do see that it's one of the most impressing stations in Europe you may decide to do a visit and leave one hour later for Amsterdam. Or head into town for a stroll leaving the luggage at the station. 

Ok, thank you for your patience with a newbie to European train travel. I was looking at IC 735 and IC 9247. As we are getting on in years, we purchased first class Global passes, hoping we would get a comfortable seat but we are disappointed to learn that after considerable expense, we are not guaranteed a seat! I think I hear you saying if you definitely want a seat, choose the mandatory requirement option but gee that is getting very expensive. The alternative seems to be to take a chance and risk having to stand for 3 hours.

I should repeat: You are worrying about things where there’s nothing to worry about. You’ll find an empty seat. 

There’s no need to do so, but you may choose a connection via Bruxelles Midi. Just 20 minutes longer but you’d board the train to Amsterdam at it’s departing station where it is completely empty. (Gent to Bruxelles is less than 30 minutes.)

Thanks Hektor.