Distance Fields

After reviewing the documentation for UE4 I’ve come across a few questions regarding Distance Fields and the overall Distance Field Atlas.
I have a lot of foliage & Meshes in my Level. I’m interested in reducing the current distance field atlas size, as its exceeding the 512mb limit (compression, 16bit).
I’ve divided my level into four smaller levels (non referenced, separate entities).

My first question is how is the Distance Field Atlas comprised? Does it include all levels or is it level specific.
My second question is how is the foliage referenced in the atlas. Does each individual foliage placed (each tree, bush etc) contribute to the overall size?