x1 is a real number which belongs to the interval (0, 1), or it belongs to the set x2 (contextless default: empty set), exactly.
This word is good for "percentage" or "fraction" (one sense). It can be used in order to construct a lujvo for "continuum" (the interval (0,1)). Generically, no further restriction is placed upon x1. In particular, it need not be rational. x2 need not be a subset of the reals. However, in particular, x2 = Set(0,1) allows x1 to belong to exactly [0,1]. x1 belongs to no set which is not a subset of (0,1) \cup x2.