Control of Documented Information in Questions and Answers (English Edition)

The revised version of ISO 9001:2015 requirements sometimes are confusing, especially regarding the 'documented information'.
What is documented information? What documented information is mandatory to keep - and what is optional?
To help facilitate the understanding of ISO 9001:2015 Control of Documented information requirements, we have prepared a collection of Documented information related questions answered by ISO 9001 experts.