Details

Split selectors onto separate lines after each comma, and have each individual selector occupy a single line.

Bad: comma-separated selectors not on their own lines

.error p, p.explanation {
  ...
}

Bad: descendent selector spread over multiple lines

.error
  p,
  p.explanation {
  ...
}

Good: each selector sequence is on its own individual line

.error p,
p.explanation {
  ...
}

Note that selectors containing interpolation are ignored, since the Sass parser cannot construct the selector parse tree at parse time, only at run time (which is too late for scss-lint to do anything with).