I suspect the definition being searched for is something is confidential if you have to keep it quiet for strictly professional reasons; my doctor is not supposed to blab that I got a tetanus booster in 1998 after stitches to my thumb in a minor accident not because anyone on the planet cares, but because an inherent part of the professional relationship is being able to trust a doc not to repeat medical stuff to random people. Secret is where you keep it quiet for non-professional relationship reasons. If the .mil didn't want you to know I hurt my hand on a secret CIA mission to Iran, and my doc blabbed that someone was hurt on a CIA mission in Iran, that's a secrecy violation that has nothing to do with the relationship between me and my doc. (I actually cut my thumb at work, boring story, not a CIA mission)

A negotiator needs to shut up about what his clients tell him, or he's not going to get clients anymore just on general principles. So thats confidential.

A negotiator doesn't have to care much about secrecy other than not getting thrown in jail. It really doesn't matter to his professional relationship if some random citizen knows what he's doing as long as it doesn't violate confidentiality.

So you're confidential to protect your own professional job, but secretive to help others keep their jobs, more or less.