Short answer: You can freely use arc42 for whatever purpose (including commercial, private, government etc.) you like without restrictions. You may not re-distribute modified versions without giving appropriate credit and sticking to the original license…
Longer answer: Creative-Commons Sharealike 4.0.
Under this license, you’re free to:
- Share: copy and redistribute the material in any medium or format
- Adapt: remix, transform, and build upon the material for any purpose, even commercially.
We, the licensors, (Gernot Starke and Peter Hruschka, the creators of arc42) cannot (and surely will not) revoke these freedoms as long as you follow the license terms.
You must give appropriate credit, provide a link to the license, and indicate if changes to arc42 were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.
If you remix, transform, or build upon material from arc42 and share the result outside your own organization, you must distribute your contributions under the same license as the original.
What this means for companies
You can copy, modify and use arc42 inside your organization without ever publishing anything.
The ShareAlike condition applies when you make an adapted version of the template available outside your organization, for example on your website, in a public repository or as part of a product you ship. Whatever you change for your own teams stays yours, there is no obligation to publish it. When you do pass the template or an adaptation on to others, credit Gernot Starke and Peter Hruschka, link to the license and indicate your changes. For purely internal use we simply appreciate the credit.
The license covers the template itself, meaning its structure, its headings and its explanatory text. Content that you write into it is yours, under your own rules. Publishing an architecture document in arc42 structure does not place your content under CC BY-SA.
See also Question K-1 (customizing arc42).
arc42 is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.