Taken from the SDK in regards to your comment.
PHP Code:
enum SolidFlags_t
{
FSOLID_CUSTOMRAYTEST = 0x0001, // Ignore solid type + always call into the entity for ray tests
FSOLID_CUSTOMBOXTEST = 0x0002, // Ignore solid type + always call into the entity for swept box tests
FSOLID_NOT_SOLID = 0x0004, // Are we currently not solid?
FSOLID_TRIGGER = 0x0008, // This is something may be collideable but fires touch functions
// even when it's not collideable (when the FSOLID_NOT_SOLID flag is set)
FSOLID_NOT_STANDABLE = 0x0010, // You can't stand on this
FSOLID_VOLUME_CONTENTS = 0x0020, // Contains volumetric contents (like water)
FSOLID_FORCE_WORLD_ALIGNED = 0x0040, // Forces the collision rep to be world-aligned even if it's SOLID_BSP or SOLID_VPHYSICS
FSOLID_USE_TRIGGER_BOUNDS = 0x0080, // Uses a special trigger bounds separate from the normal OBB
FSOLID_ROOT_PARENT_ALIGNED = 0x0100, // Collisions are defined in root parent's local coordinate space
FSOLID_TRIGGER_TOUCH_DEBRIS = 0x0200, // This trigger will touch debris objects
FSOLID_MAX_BITS = 10
};