x1=t1 is the exponential result of base 10000 (myriad) multiplied by x2=d2=k2 of -yllion(s) (default 1), to power/exponent x3=t2 (default 2).
-yllion is a proposal from Donald Knuth for the terminology and symbols of an alternate decimal superbase system. Myllion(s)(x3=2), byllion (x3=4), tryllion (x3=8), quadryllion (x3=16) and so on. See also: myriad (=suzdekyki'o).