Is the sandbox concept really that open for different interpretations? I would say no. It’s a quite obvious metaphor: it’s a framed area. You can’t miss what’s inside and what’s outside. That’s the main idea.
This implies to me that you really do not fundamentally understand the concept.
The point of a sandbox is not the boundaries, but the lack of GM driven plot. The plot, if there is one, is baked into the tables and prepared settings, not GM-presented directly.
Likewise, as to bounds, again, you've missed the point. You lightly prep the starting area, place some hooks, and maybe some prepared places. You start them off, and then, as they move, expand to keep at least one session travel around them. As you go, you add more places and more hooks and see if they bite.
There is a concept, elucidated by Marc Miller (Traveller's designer): Map only as really necessary
. But it's not just the map. It's also the setting, the tables. Change as you need, but not until you need. Marc explains it better. still...
Sandbox is about letting players experience the world in a way they
choose, enabled by a GM who makes many interesting places, and makes players always feel they're only able to experience some of the wonders. They should always be choosing which adventure they're going to pursue.
Ok, just to make it clear: have you read any of the other Year Zero games? There (and in many other games) a sandbox is A LIMITED area. And it makes sense: what’s the point of the metaphor if it’s something unlimited? A real sandbox has frames. You put something in it. Do whatever you like within that frame. That’s it.
And since Alien RPG is a part of the Year Zero line it’s relevant what a sandbox is in the other games. How hard can it be?
Of course, as a creator of a new RPG, you are free to change or twist any concept, but it helps if you show your customers what you are doing. In the other Year Zero games, the sandbox concept is very obvious. Here it isn’t. It’s never explained. If course, I see all the tables and I get why they are there, but I still think it’s not specific and useful enough.
I liked the sandbox in for example Tales from the Loop. There you can play in an 1980s version your own hometown. That’s the sandbox (or maybe it’s called adventure landscape in the English version). You can put what ever you like in it. If the entire world would have been the sandbox, it would have been to vague. For the same reason, I would have prefered more limited sandboxes also for Alien - for example a large space station, a Lunar colony or maybe an asteroid belt with several adventure sites.
It’s also a matter of how locations, more or less close to each other, are linked to each other. And you can get an idea of what you see through a map - like the Zone in Mutant Year Zero.
So this isn’t just about how to use a word, it’s also about framing in what you want to tell. If it’s framed in, it’s more of a story. The TV series Dallas is called Dallas for a reason. The city is both the frame and it’s content.
You have a Traveller background, right? I don’t. I played a lot of old school (BRP mostly) Swedish games back in the 1980s, but the past ten years or so, I’ve been more into indie or indie twisted games. Year Zero is basically an indie line with old school elements. So for me, this isn’t just Traveller with xenomorphs. It’s rather Apocalypse World with xenomorphs (that game was an important inspiration for Mutant Year Zero). I think that’s partly why you don’t get what I’m saying. Maybe this will help YOU understand concepts better, although I have my doubts...
Having all this said, it’s relevant to discus what you expect from a game and it’s concepts. I know for sure that I don’t think the Alien ”sandbox” framework is useful enough. That doesn’t mean that I don’t understand it. It’s very simple.