x1 is a Pythagorean tuning with x2 (default infinitely many) notes and a generator fifth of size x3 (default 3/2), containing a wolf fifth of size x4 (default none, unless number of notes specified).
With its default fifth size, this word is synonymous to "cuvytonci'e li ci", 3-limit just intonation; but this word can also refer to tunings with different sizes of the fifth, including meantone tunings.