[TheOperator]

I drop "Per our discussion" PURELY to put things on the record for auditing/CYA reasons. You say "it's an expression of deep frustration with a tinge of insult at it's target."

No It's business not personal. It's literally just saying that I informed you about whatever issue. I do this to inform my team that you've already been informed about an issue and you don't need to be bothered again, to trace back to where something went wrong if an incident occurs, and to protect myself by proving that I did my job.

Sure you can use "per our discussion" maliciously but it's not always like this.

[mattrp]

Let’s face it someone who has deep frustration pretty much only cares about cya so in a way these two cases are in fact highly correlated. You might also want to question why you need a cya if you don’t harbor or anticipate “deep frustration.” :)