If that's the case, when would an order ever be invalid or illegal? I suppose attempting to move a unit that is not one's own, or ordering a unit that does not exist is invalid, But even then, you say "you can take a blank board, put a single unit on Lon and say "Is A Lon-Bel valid, legally written move order" and the answer is yes or no, not "it depends where other units are"." Therefore, it oughtn't depend on even where even the single unit is. If Lon to Bel is a valid order whether a convoy is possible or not, Lon to Bel should be valid whether there is a unit in Lon or not. Lon to Bel is a valid order - but there might not be a unit in London to move (or, a unit of a different Power). The written order is always going to be valid, even if it cannot be implemented.

But I suppose my definition of a valid move order should be stated. Forgive me if I've already used this analogy. I can write 2+2=5. It is a legitimate thing to write. All those symbols exist, and we all understand what is happening. That said, suspending any 1984 references, it is a false statement. Similarly, I can SAY my mother was Harriet Tubman, but she is not. But I can construct the sentence. It has no necessary root in reality, and requires none. Even dividing by zero CAN be written, even if it cannot be done. When something is written, it must be evaluated based on accepted definitions. Then the statement can be verified against known facts. 2+2=5 is false. But are Frxhhz more likely to eat a marshmallow than a Tredkfl? It's an unanswerable question, because two of the terms are undefined. It is not true or false, it's nonsensical.

A move order is X to Y. Perhaps X --> Y or X moves to Y. It is one province, some symbol or word indicating movement, and another province. The first province is the origin, the second the destination. Neptune to G4 is a valid move order, even though it makes absolutely no sense. It cannot be translated onto the board, but the order is not invalid. The best you could do is call it irrelevant.

Even moves traditionally understood as illegal If I am England, it is acceptable for the GM to assume I am ordering my own units. Therefore if I order a different power's unit, the GM has two options. I either mistakenly thought it was mine or am cunningly attempting to use it. In the first case, the order (let's say, Par to Bur) is valid, but irrelevant, because there is no English unit in Par. In the second, the order is irrelevant because I simply cannot under any circumstance order an opponent's unit. But the order is valid anyway, even if my intention was a direct violation of the rules. If I say "FRENCH Par to Bur," is that illegal? It's illegal for the MOVE to happen, but what about the order? That depends on your interpretation of the rule about ordering other units (I actually can't find it in the rulebook. Maybe it's not there, because the whole idea is so clearly against the rules. Battleship doesn't specify you can't yell Fire and then as your opponent panics you look at where there ships are....

The only move I'd say is definitely illegal is a move to Switzerland. Unlike Neptune or G4 or Mushroom Kingdom, the rules explicitly state Switzerland cannot be occupied. Actually, even then MOVING to Switzerland is not forbidden, just occupying it. So the move is only illegal if it would cause the player to occupy it. If two players move to Switzerland and bounce, technically Switzerland remained empty because of the usual game mechanics, not because of any special rules.

Another thought, a Power cannot dislodge or support the dislodgement of its own unit. Therefore Lon to Bel might be illegal because of other units. Let's say Bel to Ruh and Hol supports Lon to Bel and ECH convoys Lon to Bel, all owned by the same Power. Mun goes to Ruh and bounces Bel. Lon to Bel is now illegal, even if Pic and Bur also supported the move.

So except for situations explicitly stated in the rules, I see no reason to declare any moves illegal, and I find the phrase invalid to be misleading. The order is fine, it just may not work, whether for that turn (Bel to Lon but no convoy) or forever (Bel to Par).