Questions where working on more than one line is a relevant aspect. In most cases that means repeating the same command(s) over several lines. – Related: the [repeated-commands] tag
Questions where (potentially) working on multiple lines is not a relevant aspect should not be tagged multiple-lines, even if they relate to line multiplicity in some way. An example would be counting the number of occurrences of a word in a file: In this case it doesn't matter whether the file has one or multiple lines.