Hi guys,
FS accesses each of the main landscape elements - terrain mesh, ground textures + autogen (i.e., land class), and scenery objects - independently. Thus, it doesn't matter whether your mesh file entries are above or below land class or scenery entries etc., though sorting won't hurt and certainly helps with being able to keep track of what add-ons you have installed. Priority is important only if you have overlapping files of the same type, for example two land class files, or two mesh files, or two scenery files with excludes, etc. In those cases the entry higher up in the scenery library menu will get priority (including for mesh files of the same resolution, which was reversed in FS9).
Sometimes add-ons come with several different folder entries in the scenery library, which need to be in specific order. It's always a good idea to check the manual/documentation for specific instructions regarding placement.
Cheers, Holger