Damn, this is really excellent. Over the course of my career I've written many tech specs, plans, one pagers, six pagers, functional requirements docs, ... etc. Each has been missing something, been too verbose, had sections that really don't integrate well. It's nice to see something a little more cohesive and rationalized.

Even if I don't ever use Arc42 explicitly in the future, this is a great reference of what things to remember and include in whatever other format of initial spec. Thanks for the overview.