I was reading through this spreadsheet posted by Cease courtesy of Prum in this thread.

Why do they list two points of diminishing returns for crit, one vs mastery and one vs power? And, how do you implement that in practice?

For example, on a Vigi Guardian, the point of diminishing crit returns in comparison to mastery is 3053, and the point of diminishing returns in comparison to power is 3237. What do I do with that information? Why would I want to go with power vs mastery?

And how do I use this information for a tank? The spreadsheet doesn't include info for Def Guardian, so what type of builds do PVP tanks use?

Don't take those crit numbers as gospel. They're meant for pve, and they assume full accuracy, pve tacticals, and sustained damage rotations. No one has done detailed calculations for most of the specs for pvp (where people take less accuracy, often use different tacticals, and often prioritize burst damage).

Anywhere from 3000-3500 crit is a pretty safe bet for all classes. Some can go a bit higher.

Believe me when I say I know this answer will be frustrating if you like to min/max. To my knowledge no one can tell you something like "3353 crit is optimal for veng juggs in pvp". I wish there were clearer answers myself, but it is what it is.