After quite a bit of digging through the SNCB website at
https://www.belgiantrain.be, I found the English-language page for their standard tickets, at
https://www.belgiantrain.be/en/tickets-and-railcards/standard-ticket
This has links to documents in French and Dutch; the French version includes the following:
QUE PERMET MON TICKET ?
• PUIS-JE FAIRE UNE HALTE DURANT MON VOYAGE ?
Oui. Vous pouvez interrompre* votre voyage dans n’importe quelle gare* ou point d’arrêt* situé sur votre itinéraire le plus court en kilomètres, et ce, dans le sens de votre destination.
Putting this through Google Translate, I get:
WHAT DOES MY TICKET ALLOW?
• CAN I MAKE A STOP DURING MY TRIP?
Yes. You can interrupt* your journey at any station* or stopping point* located on your shortest route in kilometers, in the direction of your destination.
The asterisks are links to another document containing definitions; "interrompre" (interrupt) is defined as making a stop at an intermediate point on the journey.
The significance of the "shortest route" is that in some cases SNCB tickets can also be valid via an alternative longer route (presumably faster), but if using such an alternative route it seems that break of journey is
not permitted.