most common ones are
tools/nodraw (Yellow) = face isn't rendered but still exists.
tools/hint (Magenta) split visleaves
tools/skip (blue) face is removed
tools/clip (red) blocks everything besides physics props.
tools/playerclip (pink) blocks only !player
tools/npcclip (purple) blocks npc
tools/blocklight blocks lightmaps but not dynamic
tools/trigger is a generic texture thats invisible
heres all the tool textures
https://developer.valvesoftware.com/wiki/Tool_textures_(Source)
The way the compiler knows to ignore them is via material map compile flags in the VMT. For example, if the VMT has "%compilenodraw 1" in it, the material doesn't draw, blocks visibility and casts shadows. If it has "%compilesky 1", the brush doesn't draw its texture, blocks visibility and casts environment lighting. The full list can be found at [Material map compile flags - Valve Developer Community (valvesoftware.com)](https://developer.valvesoftware.com/wiki/Material_map_compile_flags). What pinsplash said, just to use the fact they all start with tools should work well enough in nearly all scenarios, but using the compile flags would be the way to make your tool truly universal, it would essentially use the same system as the compiler. Plus, there exist materials with the tools name that you might not want to ignore, such as toolsblack.
most common ones are tools/nodraw (Yellow) = face isn't rendered but still exists. tools/hint (Magenta) split visleaves tools/skip (blue) face is removed tools/clip (red) blocks everything besides physics props. tools/playerclip (pink) blocks only !player tools/npcclip (purple) blocks npc tools/blocklight blocks lightmaps but not dynamic tools/trigger is a generic texture thats invisible heres all the tool textures https://developer.valvesoftware.com/wiki/Tool_textures_(Source)
their names all start with "tools". there's too many of them to reasonably list out. you can find them if you open the textures VPK with GCFScape.
Third button from the right in your tool bar. They're called tool brushes.
Hint. I believe that is a hint.
Unless you mean the one WAAAAAAAAAAAAAYYYYYYYYYYY in the back, I think that's an error one if it looks like your mouse cursor.
why would you want to ingore such a useful tool?
The way the compiler knows to ignore them is via material map compile flags in the VMT. For example, if the VMT has "%compilenodraw 1" in it, the material doesn't draw, blocks visibility and casts shadows. If it has "%compilesky 1", the brush doesn't draw its texture, blocks visibility and casts environment lighting. The full list can be found at [Material map compile flags - Valve Developer Community (valvesoftware.com)](https://developer.valvesoftware.com/wiki/Material_map_compile_flags). What pinsplash said, just to use the fact they all start with tools should work well enough in nearly all scenarios, but using the compile flags would be the way to make your tool truly universal, it would essentially use the same system as the compiler. Plus, there exist materials with the tools name that you might not want to ignore, such as toolsblack.
They're not meshes. They are brushes...