Tooltexture

Todo: find out what else this parameter is also used for

Used for blend texture previews in hammer.

VMT syntax example

"WorldVertexTransition"
{
	"$basetexture"     "dirt/blend1_mountain_dirt_a1"
	"$basetexture2"   "dirt/blend2_mountain_dirt_a1"
	"%tooltexture"      "preview/blend_mountain_dirt_a1"

	$bumpmap           "dirt/blend_mountain_dirt_a1-ssbump"
                "$ssbump"   "1"
 
	%detailtype desert1
	"$detail"    "dirt/blend_mountain_dirt_a1_DM"

	$surfaceprop      dirt
	$surfaceprop2    rock
}

The texture in preview/

This is useful for showing what the 2 blend textures are in a single image, without this it will just show your basetexture on it's own thus not showing it's second blend.