(sorry for necro, but me and I'm sure some other people sometimes refer back to this thread occasionally when doing detail stuff, since there's yet a page on the VDC.)
So I've done some testing on the world space masks and how exactly they line up with your hammer grid.
Scenario One: Square Map
This is pretty simple. Your tint/type mask is a square texture, and Source creates a bounding box around your map. If your map is square, this works out pretty simple and your world space mask will cover the entire map. It is aligned to the bottom left of the bounding box created (when looking down from the 2d top view in Hammer).
Scenario Two: 'L' shaped map
This is pretty similar to the square map - Hammer creates a bounding box around your map and aligns the world space mask to the bottom left. Even if your map has the corner of the 'L' in the top right, your world space mask is still aligned to the bottom left corner of your maps bounding box.
Scenario Three: Rectangular Map
The world space masks are still aligned to the bottom left of your maps bounding box, however it's important to note that they do not stretch aspect ratio!
Scenario Four: Unconnected sections
If your map has different sections (completely separated by void/skybox), a bounding box is created over your whole map and the tint mask is still aligned to the bottom left.
Scenario Five: Map with large amounts of 'void' at the borders.
So I've talked a lot about the bounding box of your map, however its also important to note that this bounding box is only created from the corners of textured displacements - the void does not count! This means if your map has a box skybox from one corner of the hammer grid to the other, but only has one small displacement in the middle, the bounding box only counts for that displacement.
Scenario Six: 3d skybox next to your main map (not above or below)
If there is a sky_camera in part of your level, that part of the level does not count towards the bounding box created. This means you don't have to put your 3d skybox under or below your level and it's perfectly fine being next to it. It's also important to note that I'm not sure exactly how the world space masks line up with 3d skybox, seems to be a bit different from a regular level (but who's gonna put detail sprites in the 3d sky anyway lol).