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.
Best Answer
Moving either version should break the spell, at a DM's discretion
As Jack points out, Glyph of Warding used to have the same language. The designer intent of this language (for Glyph of Warding) as shared by Mike Mearls in a tweet was:
Presumably this means; "If you can move the thing it is cast on, then it is an object and thus cannot go more than 10 feet from the casting point"
As Thomas inferred, in practical terms the surface is the option that cannot be moved, so you need the movement clause for only the object case. However, a table qualifies as an object in the game, and can be moved (with some difficulty as it is unwieldy), and this caused problems and confusion with the wording.
Glyph is much more heavily played1 and thus has been a source of questions. Consequently, its language has been errata'ed in 2016 to include the following sentence:
The errata solved the issue by making sure movement breaks the spell regardless of what it is cast on, instead of explicitly declaring surfaces only as those that cannot be moved. Now, why did they not apply the same errata to Symbol?
One could argue that Symbol is much higher level, and therefore moving it is an option that was consciously left in. Because frankly, you can upcast Glyph to any level and cause effects with it that are just as nasty as Symbol's, so what is the point of Symbol at 5 times the gp cost, to conserve an extra slot when prepping the effect?
Or, leaving out the movement clause could simply have been an oversight, as Symbol did not come up in player questions. We cannot know for sure.
Strictly as written, you could move it if you chose the surface option. That makes little logical sense, as you point out yourself. So, you as the DM need to decide how you want this to play out.
Personally, I think the lack of errata was just an oversight, and you should play it as if it would also break on surfaces when moved, but you can decide against it and follow the text.
P.S. If you really wanted them to bring pre-cast symbols along, they could cast them on the surface walls of a portable hole or Demiplane. I think these are pretty "gamey" tactics, though.
1 90% of play happens in tier 1 and 2, where players can use Glyph but not yet Symbol.