Used when a line has one or more whitespace characters directly before the line end character(s).
This message belongs to the format checker.
The PEP 8: Programming Recommendations mentions trailing whitespaces on a line only in context of split strings:
Don't write string literals that rely on significant trailing whitespace. Such trailing whitespace is visually indistinguishable and some editors (or more recently, reindent.py) will trim them.
PyLint recommends not to have trailing whitespace on any line.