The difference between the flags is their effect when used with 0x8000. With the 0x18000 flags, the weapon will use its trailcylradius and trailrandangle properties (as well as trailthing and range) to give the trailthings a semi-random effect.
JK's force_lightning is well-known example. Notice that each lightning piece veers off from the projectile's path (according to trailrandangle), but a piece never goes farther astray than the trailcylradius distance. Also note that the engine "connects" the trailthings so that the ending point of the last trailthing is the starting point of the next.
As with 0x2000, elementsize and range must be used for this flag.