CommCare is an open source software. This page provides some detail on what that means.
Software is open source if the original source code is made freely available and can be redistributed and modified. Dimagi releases its software under the most permissive of Open Source Initiative software licenses (BSD and Apache v 2.0). These licenses grant organizations complete freedom to re-use, redistribute, and reproduce Dimagi's software without consulting Dimagi.
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You can find more detail about the CommCare code base in the Advanced section of the CommCare help site and the Developers section of the CommCare Website.