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),
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