First of all you need to know that for foliage you can use a ‘Static Mesh’ or a ‘Static Mesh Foliage’. They are treated a bit different in the foliage tool as you’ll see later:
I have an empty main level wich containes two instanced levels under the persistent level.
The first of them L_Holzi containes all my foliage. Right now I have selected the Persistent Level as my active level (wich is marked in bold blue in the levels tab). It does not contain any foliage. That’s why the foliage tool is showing no ‘Static Mesh’ objects but only my 5 ‘Static Mesh Foliage’ objects that I have used in my other level L_Holzi.
You only can see ‘Static Mesh Foliage’ objects from other levels.
(like from my L_Holzi level in this case). But:
You can’t see regular ‘Static Mesh’ objects from other levels.
If you double click on the level wich contains all your foliage, you can see all of it in the foliage tool. ‘Static Mesh’ and ‘Static Mesh Foliage’ objects are visible now.
If your have not explicitly saved your static meshes as Static Mesh Foliage (wich I also never did before), no regular ‘Static mesh’ objects from other levels will be visible in your active level.
If you have saved your static meshes as Static Mesh Folitage, they will be also visible in your other levels.
You can simply create a ‘Static Mesh Foliage’ object out of your foliage tool list by right clicking > Save As Foliage Type.
Static Mesh Foliage objects from other levels will show a ‘0’ for the instance count, because you are not using them in your current level. In my case I use the English Ivy 17 times in my L_Holzi but not in the Persistent Level. You can see the that when you hover over it.