I've croutonized two of my chromebooks with debian: one jessie, the other one sid - simply to access latest packages.
Would "sid" now become the testing?
Would "sid" now become the testing?
sid is then branched into a new codename which moves through the testing phase and then stable.
"Testing" and "stable" are rolling names which always points to the branch currently in the testing or stable phase, respectively.
(Well, simplified. There's also the experimental branch, but that's not something you can run. Then there's oldstable which is the phase after stable.)