What `relative` means is not very clear. The following is copied from LAPACK:

> RCOND is used to determine the effective rank of A. Singular values S(i) <= RCOND*S(1) are treated as zero.

We can say "Sets the reciprocal condition number (`rcond`). All singular values smaller than `rcond * sigma(0)` are treated as zero, where `sigma(0)` is the largest singular value."