What’s the difference between the surface fills “Monotonic” and “Monotonic Line” ? The latter appears to be a Bambu extension, and - according to the source code as FillMonotonicLineWGapFill - is its own thing, inheriting from Fill rather than inheriting from FillMonotonic or FillRectilinear as one might expect.
Alas, I’m not deep enough into slicer software to be able to determine the difference just by wading through the function definitions; it’s all black magic to me at the moment.
best info is at Bambu Lab wiki.
[Bambu Lab Wiki](https://Bambu Lab Wiki)
bottom of page shows the difference.
-Uman
Thanks for the reminder that there’s a wiki there 
For reference, the link to the fill types page is this: Infill Patterns | Bambu Lab Wiki
Monotonic Linear looks like ■■■■ compared to Monotonic. I wonder why they bothered to modify the software like that. Perhaps there’s some situations where overlap with perimeters is undesireable?
There’s another difference, likely a bug in fact. When using monitonic line only the topmost layer is monotonic, the other top layers are not. This can reveal ugly lines depending on the printed object and filament. From my experience it is very visible with silk filaments.
Also I think it is fixed on the softfever fork but don’t quote me on that, I didn’t try it myself.
I don’t think it’s a bug. After experimentation, there is no setting (in Bambu Studio, at least) that lets you control the infill pattern for subsurface solid layers (second, third, second-last, third-last, etc).
From Infill Patterns | Bambu Lab Wiki
compare[d] to Monotonic, Monotonic linear infill has no overlap with the perimeter, some seam may appear on the surface, but will not has redundant material in connected areas.