Software is available under an open source license, and support is provided for a fee.
The Support model is one where the software is available under an open source license, but in order to have the company available to answer questions, assist in implementation, etc., you must purchase a support contract.
Who Uses it?
When should it be used?
This is typically an early way to bring in revenue for an open source project. If/when the company or project needs to scale, it tends to transition into another model.
What kinds of monetization is possible?
Does this model help create a Sustainable Free and Open Source Community?