This question brought back memories of a situation that occurred to me about 40 years ago. (I haven't played much Diplomacy since then.)
I was playing France, worried about England and Germany, and England made an "alliance" offer to me arguing, "You will have 5 supply centers at the end of 1901, Germany will have 5, and I should have 5, to preserve the balance of power. Will you support me into Belgium against Germany using the army that you moved into Burgundy in spring, 1901?"
I "temporized" by saying that I would support a British fleet into Belgium, but not an army. So in my Fall, 1901 orders, I wrote, "A Burgundy supports fleet only, N Sea to Belgium."
England's orders read, "A Yorkshire to Belgium, F N Sea convoys." This was against my express intent, and after an argument, we took the issue to the host/organizer (who was playing Turkey). The host ruled into favor of England, who argued, "The army went to Belgium via the fleet N. Sea, and the supporting French army couldn't know until after the fact whether it was the fleet or army that occupied Belgium."
To this day, I think that the host (and England) were wrong. Were they?