None
No business model
This is a common archetype for Sustainable Free and Open Source Communities. The business model is to not have a business model. That is okay, because not every sustainable open source project needs or wants a business model.
In this case the project has set up means and rules of collaboration, has a formal membership, usually without a formal position of authority or any other hierarchy and does not deal with monetary contributions.
Sustainability in this non-model focuses on collaborator growth (quality and quantity) and shared responsibility to address its biggest risk: contributor churn.
Who uses it?
When should it be used?
In the early phase of projects when it’s not clear that the idea/implementation is a viable option in the free software ecosystem. Or when it’s clear from the beginning, that the target audience is rather limited in size.
Sometimes this model is also used as a way out of entrenched corporate influence. Incubating a new project, while blacking out monetary interests, trying to supersede the fought-over-project by technical means. Kind of a fork of the problem space (not the technology), resetting the relationship of the involved parties. A prominent example for this would be systemd or the GNOME desktop environment.
What kind of monetization is possible?
Cash flow is avoided in this model because there is no legal entity behind the project (just individuals) that can handle the added complexity (control, reporting, taxes etc.) that monetization brings.
The individuals involved cover the development costs for the project by donating their time. Infrastructure costs are avoided by utilizing free to use plans of services like gitlab/github, travis-ci/circle-ci, netlify etc. or by donations of goods by members (like paying for a domain name etc.).
Does this model help create a Sustainable Free and Open Source Community?
Yes. This model can be applied for a very long time as long as the community sucessfully addresses contributor churn.