Every field is optional. If you leave them all blank, you will get a page with ALL prefabs. If you only select custom textures, you will get a listing of all prefabs with custom textures. If you want to find all prefabs for Half-Life, that don't have custom textures, and are in .qle format, then you can very easily by selecting below. All rights reserved.
Prefabs are property of each individual author.
For more guidelines on how you can use the prefabs in your levels, etc please contact the author personally.