Note: You’ll need an available diplomat for this.
Is Automation Efficient?
Automated diplomats can only do the Improve Relations action. They’ll aim to reach 200 points in the “Improved Relations” opinion modifier, but will ignore the actual opinion value of their target nation. This means that they can continue improving relations with a target even though they already have a max opinion of you. This can be inefficient. Also, when you have an empire spanning continents, the envoy travel time for diplomats will become more significant. It may take them months to switch between targets. That said, targeting outraged countries can be a convenient way to delay coalitions from forming. That’s the only automated assignment where diplomats will switch to their next target once their current one loses their “outraged” attitude.
How Do Diplomats Pick Their Targets?
This depends on where you’ve assigned them in the automation screen.
Neighbors are countries you share a land border with, including borders of your directly owned colonies. Subjects include your vassals, junior partners, colonial subjects, and tributaries. Outraged countries have the “outraged” attitude, which happens when you’ve amassed a lot of aggressive expansion. Allies are strictly those who you’ve an alliance with. Threatening countries are those who’re more powerful than you.