  Streams of event weights in particle-level Monte Carlo event generators are a
convenient and immensely CPU-efficient approach to express systematic
uncertainties in phenomenology calculations, providing systematic variations on
the nominal prediction within a single event sample. But the lack of a common
standard for labelling these variation streams across different tools has
proven to be a major limitation for event-processing tools and analysers alike.
Here we propose a well-defined, extensible community standard for the naming,
ordering, and interpretation of weight streams that will serve as the basis for
semantically correct parsing and combination of such variations in both
theoretical and experimental studies.
