What it actually is
A single-needle machine puts down a line roughly a third of a millimetre wide. A standard liner is three to seven needles soldered in a cluster and lays something two to three times that. Everything else about the process is the same: same depth, same ink, same skin.
That is the entire technical difference, and it produces the whole aesthetic. Less material per line means more lines can sit close together before they read as one mark, which is what lets a fine line piece carry detail at a size that a bold outline could not.
What it is good at
Script, especially handwriting. Botanical linework. Small figurative work under about four inches. Anything where the subject is delicate and you want it to stay delicate rather than becoming a strong graphic shape.
It is also the right answer for a first tattoo more often than people expect, because the sessions are short, the healing is quick, and a two-hour commitment is a reasonable way to find out how your skin behaves.
What it is bad at
Scale. A fine line piece the size of a forearm panel is a fine line piece that will look like a smudge at ten feet, because there is nothing in it heavy enough to carry the composition. Above about six inches, Sloane will usually propose mixing weights, which is fine line with a few heavier anchors, and it is almost always the better drawing.