The ZDoom wiki mentions the scrollbits being dependent on the type of scroller.
However, if I choose any of the two options Displacement (1) or Accelerative (2),
Spoiler:then nothing scrolls.
There doesn't seem to be any information in the wiki as to how those two options are employed,
at least I could not find any.
The scroll happens If I choose none of the options, leaving it as 0, or if I choose the option 'Scroll by linedef', which
is like the BOOM special 253, where the speed of the scroll is determined by the length of the linedef.
Concerning 'Line Tag' , I posted it as a bug in GZDoom Builder 1.14