ADR is an abbrevation for "Architectural Decision Records". ADRs are a form of "Why?" documentation.
- when you architecture documentation is lacking
- high turnover of senior team members
- when you have incomplete and vastly out-dated documentation
- when you split your DevOps documentation in What/How and Why
- when you need a documentation style for a new architecture board
- when your team does not have access to a supporting architect
There are many templates out their with which you can start. For example https://github.com/adr/madr