Line_SetIdentification

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121:Line_SetIdentification (lineid, moreflags, reserved1, reserved2, lineid_hi)

lineid: Identification number for this line 
moreflags: Allows setting of a the line flags that don't fit in the flag word anymore.
lineid_hi: High byte of the line id

Used to identify this line for certain specials and ACS commands.

The following bits are defined for moreflags:

1: defines a zone boundary for sound environments.

2: defines a railing. A railing has the lowest 32 map units blocked but is passable above that.

4: block floating monsters

8: clip mid textures to floor and ceiling

16: wrap mid textures so that they fill the entire height between floor and ceiling. This implies clipping them to floor and ceiling.

32: 3dMidtex: Treats the mid texture on this linedef as a part of solid geometry. The texture part of such a linedef blocks actor movement (like a railing) and players and monsters can walk on such textures. Linedefs with this flag set can be attached to moving sectors via the Sector_Attach3dMidtex action special. (development version only)

64: performs a check whether a switch is reachable for the player before activating it. If a switch is too high or low or completely in the ground or ceiling it won't activate. (development version only)

How to use the high byte in lineid_hi:

Parameter 1 has the low byte, and parameter 5 has the high byte, so the tid is parm1+(parm5×256). This lets you affect lineids higher than 255. For example, if the lineid tid of 4 then the low byte is 4 and the high byte is 0. But if the lineid is 3027, the low byte is 211 and the high byte is 11 (11×256 + 211 = 3027).

Note: For the original Hexen maps, or any maps using the Hexen MAPINFO format, the 2nd-5th parameters of this special are ignored. This is to restore compatibility with classic maps which may have set these parameters even though they weren't used at the time. To ensure that this special works as expected, make sure to use ZDoom's new MAPINFO format as defined here.