This is a static linedef special which sets Boom-style friction on tagged sectors. Its Doom-format special ID is 223 and when used under that format, amount is 0, so it will use linedef's length as parameter.
- tag: tag of sectors to affect. They must have the "friction" generalized sector special flag enabled (have 256 added in Doom format or 2048 in UDMF format to special).
- amount: optional amount. < 100 is sludge, > 100 is ice. If 0, this will use linedef's length.
In UDMF, you can also set a sector's friction field directly and avoid using this special.
The friction effect can be turned on or off by using the texture/type floor changing specials, similar to how you can change a sector to be a damaging floor or not.