%keywords
Keywords allow you to filter textures inside Hammer Editor's material browser. Since the default material search only filters by material name or folder name, you can use these keywords for cases where textures may fit together, but are not in the same folder.
For example, 
Half-Life 2 uses the keyword "Combine" to filter all metal, glass, computer and other combine related materials that would be all over the material folders of the game.
You may also add a keyword named after your map or mod to be able to find all custom materials with ease.
VMT Syntax Example
"WorldVertexTransition"
{
"$basetexture" "nature/cliffface002a"
"$basetexture2" "nature/dirtfloor004a"
"%tooltexture" "nature/blendcliffdirt001a_tooltexture"
"$basetexturetransform" "center .5 .5 scale 1 1 rotate 0 translate 0 0"
"$basetexturetransform2" "center .5 .5 scale 4 4 rotate 0 translate 0 0"
"%keywords" "wasteland,c17"
}
Entering "wasteland" into the keywords field in the hammer texture browser would show only textures with the "wasteland" keyword. This is useful for categorizing your textures for easier finding.
Note:To have more than one keyword, separate the keywords with a comma.Used Keywords
The following keywords are used in Source games:
Half-Life 2
- c17citadel
- c17downtown
- c17industrial
- c17sewers
- c17trainstation
- coastline wasteland
- combine
- wasteland
Portal
- portal
Team Fortress 2
- tf