Question about the criterion to define instability or stabilty

Hi everyone.

My question maybe is some basic but very important. In FLAC Manual (Factor of Safety) is the following text:

“The detection of the boundary between physical stability and instability is based on an objective
criterion in FLAC that decides whether the system is in equilibrium or a state of continuing motion.”

Can you say me please which are these criterion if I used ofr example Solve FoS?

In the practice, I put histories of displacement, velocity, unbalanced forces, etc… and run the model. Then I see if the histories with the plasticity indicators if the model have instability or not (using Strength Reduction Method “manually” in FLAC). I know too that exists some velocities to define more or least instability or stabilty (could be 1e-5 or 1e-6 in FLAC/FLAC3D, obviously verifing with some histories previously). But which is the internal criterion in FLAC to define stabilty or instabilty? Are these criterion used in FLACSLOPE for example? Thanks in advance.

On pages 1-7 through 8 of Factor of Safety volume(FLAC8.1 MANUAL), there is a section discussing the procedure for implementing the strength reduction technique in FLAC and what criterion is used.