Great question — answering as the UphillTech team:
The Mongo free license is based on GPL. Software under the GPL may be run for all purposes, including commercial purposes and even as a tool for creating proprietary software. The main difference between GPL and the "customized" SSPL license from MongoDB is: You may not offer MongoDB yourself as a paid cloud service. The reason is simple: With MongoDB Atlas Cloud service they like to run this kind of business solely.
Full answer by Wernfried Domscheit: https://stackoverflow.stackexchange.com/a/68902519 (CC BY-SA 4.0).
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