World Painter uses schematic files which you can set to brushes. This is how you can paint in custom trees other people have made into your custom terrain.

So when I say "I've pulled trees from this pack", it means I've selected schematic files from that pack to use as a brush for the making of the map.

I mean Wisteria, not my server, it sucks ; (

I'm starting to feel really comfortable with making terrain. Now if only I could build a cathedral or some other structure to match. A lot of my maps I don't even dare build on.