This function rounds the values in its first argument to the specified number of significant digits, using IEC 60559/IEEE 754 ``round towards zero'' mode. This is an alternative to the round function, which rounds toward nearest, ties to even. This is designed to be used internally by unf3 and unf4 (though the original implementations do not seem to actually use the function). unf5 uses round instead.
Rounding toward zero assures that signifz(signifz(x,digits=m),digits=n) = signifz(x,digits=n) for $m>n$, an important property for creating approximate fingerprints. It can, however, produce more rounding error than rounding toward nearest. The maximum log relative error (LRE) for the former is (digits-1) while the maximum LRE for the latter is `digits'. Hence, you may wish to use one more significant digit with signifz than with signif.