Stable keyword in Gaussian software

[skpang~at~ctimail.com]

Dear all,
 Gaussian software has a "stable" keyword to test the stability of a
 single-determinant wavefunction with respect to relaxing various
 constraints.
 However, I cannot find this function in some quantum chemical
 calculation software such as NWChem. (if I have not overlooked!)
 Furthermore, I know that many users seldom use this function.
 What is the purpose of this "stable" keyword? What are the situations
 which the "stable" keyword is required?
 Thank you for your attention!
 Patrick
 My e-mail address is : skpang~at~ctimail.com