where X is is one of Iterate, Linear, Bayer, Eliminate. These are described in the documentation node for saturate.
The Rees algebra S(M) of a submodule M of a free module (most importantly, an ideal in the ring), is equal to the symmetric algebra Sym_{k}(M) mod torsion. computing this torsion is the slow link in most of the programs in this package. The fastest way to compute it is usually by saturating the ideal defining the symmetric algebra with respect to an element in that ideal.