Clarification on instruction semantics in Customizing Copilot docs

[tspascoal]

The content about custom instructions in Customizing Copilot is great, but there are few missing details on the semantics that would benefit everybody.

.github/copilot-instructions.md

Where are these instructions taken into consideration besides chat (I assume with the exception of /tests and in code review) ? Are they also taken into consideration in code completion? other places?

github.copilot.chat.codeGeneration.instructions

Is this also applicable to code completion or just chat (while generating code)?

What happens when this is used in conjunction with .github/copilot-instructions.md ? Are both instructions considered (union)? or one wins over the other? (If yest, which one)