It might matter. Typically the lowest numbers have highest priority so if you have some other landclass with a higher priority it might override the lower priority version.
BUT I'm not sure that will happen in this case, as I don't know what is contained in Megascenery. It may not contain landclass at all.
As to why GEX is global, it's because part of the textures are shared between areas.
FS divides the world up in sections, each with its own textures.
So you will have separate textures for American farms, European farms (which are typically smaller and have more distinct borders), Asian farms (rice paddies for example), etc. etc.
But some of those textures are shared between these areas. Rocky mountain slopes for example, and certain snow and desert textures. If GEX modifies those from the defaults, you'd see those new versions everywhere.
But the main purpose of the product is to cover north America (with some overlap into central America no doubt, as the coverage areas don't follow national borders precisely).