The party will soon be negotiating with an enemy; if they are smart, they will make sure that it happens at a neutral location. One of the enemy's henchmen can cast Symbol, and he would like to bring a precast one, possibly more, to the negotiation to use offensively if things go bad (one minute casting time means it is unwise to cast during a fight, but it does not require concentration and lasts until dispelled or triggered, so it is a perfect spell for precasting).
I have never used Symbol before (as a player or DM) or had it used at my table. The spell offers two options, but I can't understand why anyone would ever choose the second one, so I suspect my understanding is incorrect.
The Symbol spell says:
When you cast this spell, you inscribe a harmful glyph either on a surface (such as a section of floor, a wall, or a table) or within an object that can be closed to conceal the glyph (such as a book, a scroll, or a treasure chest). If you choose a surface, the glyph can cover an area of the surface no larger than 10 feet in diameter. If you choose an object, that object must remain in its place; if the object is moved more than 10 feet from where you cast this spell, the glyph is broken, and the spell ends without being triggered...You decide what triggers the glyph when you cast the spell...You can further refine the trigger so the spell is activated only under certain circumstances...Once triggered, the glyph glows, filling a 60-foot-radius sphere with dim light for 10 minutes, after which time the spell ends. Each creature in the sphere when the glyph activates is targeted by its effect, as is a creature that enters the sphere for the first time on a turn or ends its turn there.
My thought is that the enemy's henchman will prepare a Symbol using the 'surface' version on a small object that can be easily thrown (though not a bouncy ball), and set to trigger under the circumstances "activate as soon as I and all of my allies are more than 60 feet away". In fact, he could potentially prepare multiple of these symbol-bearing objects (with different symbols to avoid them suppressing one another), with the only limitation being the financial resources I as the DM assign to the NPC to cover the material component cost. While a 7th level spell is his highest cast, these seem pretty safe to carry around as long as they are not misplaced, so this could have been done days in advance, and not for this particular negotiation.
The spell can be cast either "on a surface" or "within an object", but the "within an object" version ends if the object is moved (like a glyph of warding). Since the caster wants to bring these to the negotiation, he will obviously choose the first option - "on a surface", but the surface of an object that can easily be moved and thrown. The examples for "on a surface" (a section of floor, a wall) might imply that this version can't be moved either, but "a table" can obviously be moved and is in fact one of the stated examples of "an object". Further, while the "on a surface" version has a maximum size, it does not have a minimum size, so it seems like that version can be placed on as small and transportable an object as desired.
My current understanding is that the "on a surface" version of the spell is clearly superior to the "within an object" version, not just for my desired use case, but in just about any case. The surface version could be on an object but doesn't have to be, but the within version must be on an object. The surface version could be on the surface of an object that can be closed to conceal it but doesn't have to be, but the within version has to be on an object that can be closed. Neither version has a lower size limit, and while the surface version has an upper size limit so does the within version when factoring in that it must be placed on an object that can close. The surface version can be moved without deactivating it, but the within version cannot.
Given the clear disparity in usefulness between the two versions, I have to wonder why anyone would use the within version. And wondering that makes me wonder whether I am understanding the surface version correctly and can have my NPC use it the way I plan, RAW.