Each linedef has a number of attributes which can be set for special purposes. Brief descriptions of these attributes are given below. For more complete descriptions, refer to [UDS:4-3-1].
Impassible: Players and monsters cannot cross this line.
Block Monsters: Monsters cannot cross this line.
Two-Sided: In general, this should be set if the line is two-sided, and not set otherwise. Hellmaker normally sets this flag correctly; only change it if you are sure you want to.
Upper Unpegged: Normally, the upper texture is drawn from the bottom up. Setting this flag makes it draw from the top down. See [MTAUA] for a complete discussion.
Lower Unpegged: Normally, the lower and middle textures are drawn from the top down. Setting this flag makes them draw from the bottom up. See [MTAUA] for a complete discussion.
Secret: The line appears like a normal wall in the automap. (This has nothing to to with the SECRET ratio at the end of the level - that depends on the secret SECTOR attribute).
Block Sound: Sound originating from the player normally fills the sector he is in, then travels from sector to sector, activating any (non-deaf) monsters. The sound will be blocked when it hits a second linedef on a sector boundary with this flag set.
Not on Map: This line will never be shown on the automap.
Already on Map: This line is shown on the automap even before it is seen.
