The Add-on.xml directs which loads 1st.. Lower is first (or bottom up), in the add-on which is specified at 153 then higher loads so it is placed right above - and both of these will load before scenery 154.
(Orbx central or Lorby's if you run - will probably set each to different layers and OC will create a backup of the original in the add-on folder - then create it's own xml file with two layers.. but this is where things can get messed up).
For me sim, I want tongass to be loaded between layer 152 and right below whatever is at 154. (which would be one of the Orbx region entries (since Tongass is supposed to right below and Orbx Regions or Airports , but right above of any Orbx openLC landclass entries of you have openLC.
Sometimes a double layer can mess things up - sometimes not.. sometimes it doesn't matter, but sometimes it sure does.
Just try it... Open the add-on.xml with notepad and alter the layer #.. then save then load the sim and see where it is is placed in your scenery library. (Then move it to another layer# in the xml.. and you can place items up or down in where is appears and gets loaded.. (But always work from the bottom up.. you're building a pyramid.. base to the top.. You can play with this stuff, - just remember that for your sim, 153/154 may not be correct.. all depends what you have below.. you actually might have to drop it down for correct placement.
Tongass Higher sits right above Tongass Lower.. and these two entries in the add-on must be layered(#) to be right below any Orbx regions and airports, but above any open LC items.. (Hence the exact same layer - so they sit together in the 153 slot).
Hope this helps understand some of the methology of layering - using the add-on.xml layer line. So the layer # is like one of stairs of the scenery library, even as the scenery.cfg file - each area/entry has it's level or stair. (All layers or levels start at the bottom and work up).
And in the sim's scenery library, is the only place where you see all of the scenery entries plus all of the scenery add-on pieces together as it is directed to be layered.. Now the scenery lib entries (scenery.cfg) you can move around but not scenery that is loaded by add-on.xml - their layer # is hard coded in the addon.xml. (then you play with that "hard coded" layer # to get those scenery entries to fit in at the exact correct layer in the over all scenery library as viewed in the sim scenery lib.