In PolyFlags you have to enter a number, what follows is a list of the meaning of all the numbers.
To give a brush two or more properties at the same time, you need to make the sum of the numbers of both properties. For example if you want a Translucent + TwoSided brush, you have to use the number 4 + 256 = 260.
After you entered the number you need to rebuild to see the change.
Here's the list. Flags with no effect are left out:
|0||Default||Use settings from Surface Properties for each individual surface of the brush.|
|1||Invisible||All surfaces of the brush are invisible|
|2||Masked||Use for palettized textures to make the first color in the palette (in Paint Shop Pro at least) invisible, see Color Blending|
|4||Translucent||Darker parts of the textur become transparent, see Color Blending|
|16||Force View Zone|
|64||Modulated||Greyer parts of the texture become transparent, see Color Blending|
|128||Fake Backdrop||Parallax SkyBox visible through surface|
|256||Two Sided||Surfaces display their texture from both sides|
|512||U-Pan||Texture moves in U direction, change speed in ZoneInfo or LevelInfo → ZoneLight|
|1024||V-Pan||Texture moves in V direction, change speed in ZoneInfo or LevelInfo → ZoneLight|
|2048||No Smooth||Square pixels|
|4096||Special Poly||Doesn't do a lot|
|32768||Low Shadow Detail||Reduces shadow detail; in conjunction with High Shadow Detail (below), makes extra low shadow detail|
|524288||Bright Corners||No dark shadows at the edges of the surface|
|1048576||Special Lit||Surface only gets light from Special Lit lights|
|2097152||No Bounds Reject|
|4194304||Unlit||Surface ignores lighting and is fullbright|
|8388608||High Shadow Detail||Enhances shadow detail|