This category covers anything with a run time (by our calculation!) of less than 20 minutes.
(Our calculation is based on word count, assuming 1000 words on the page takes 6 minutes on the stage. Of course this varies enormously.)
At the lower limit, these verge on stand-up comedy sketches, at the upper limit there are some short complete plays.
The category includes all forms of material - so, with no other constraints, it includes the short verse plays, short plays with music, short monologues, and so on.