I won't spoil any of the opinions Adam and I have in store, but I was reviewing the rules per Michael's reference, and found this gem:

"The province to which a unit is providing support must be one to which the supporting unit could have legally moved to that turn." I then considered it and thought up this conundrum:

Let us say there are 3 French units:
A Pic
A Bel
F ECH

There is a unit in Lon, it is not French, and it doesn't matter whose it is because it is holding throughout these examples.

A Pic to Lon
A Bel supports Pic to Lon
F ECH convoys Pic to Lon

This appears as though the Bel support is irrelevant (I use relevant and irrelevant in place of valid/invalid when speaking of supports, and the reasoning will be explained in Adam's post). A shrewd player might point out, though, Bel "could have legally moved to" Lon via ECH. A wise GM would say, no, because ECH can only convoy one unit, and therefore the French player could NOT legally do both moves to Lon, and therefore the support is irrelevant.

What if there is also a French fleet in Nth? Now we can:
Pic to Lon
Bel to Lon
ECH convoys Pic to Lon
Nth convoys Bel to Lon.

Indeed this is wholly legal. Both units can be moved to London. So Bel should be able to provide support. The advantages are obvious, now Nth needs to be dislodged to disrupt the convoy which prevents Bel from reaching Lon for the support to be irrelevant. This requires more than one unit just cutting support....

I guess it depends a lot on the definition of "could." Technically if a foreign fleet in Nth is present, the player could argue the unit in Bel COULD have been convoyed, and therefore the support is relevant, even though it was not. Or the GM could require the convoy to actually have been ordered. Is it that the unit possibly providing the convoy was present, or the the unit possibly providing the convoy actually order it? Once the convoy order is published, the unit COULD move across the water, if it was ordered. But it ordered to support. But it could have gone there, so the support must be relevant, technically speaking.