Open Source
Axon products have different licenses and accept different kind of contributions

AxonIQ Open Source License - Commercially licensed software
The Message router and event store. The code is public, and anyone can use the Axon Server for free. Although, contributions are taken as suggestions or starting points for new features.
Quick Summary
You can use freely this software but have certain conditions to follow.
Can
Ability to use, copy / reproduce the work freely in unlimited quantities
Ability to make use of the work for commercial purposes for a fee but cannot be sublicensed to others
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Ability to distribute the work to third parties freely, in unlimited quantities
Cannot
Describes the right to charge the software owner or contributors for direct or indirect damages.
Describes the rigth to use the original licensor or contributors' names, trademarks or logos (i.e. for advertisement, promotion)
Ability to modify / combine the work with others and create derivatives
Ability to license the work, including possible modifications (without changing the licence if it is copyleft or share alike)
Ability to place additional warranty, services or rights on the software licensed (without holding the software owner and other contributors liable for it)
Must
Describes whether the original copyright and attribution marks must be retained
Include the full text of the licence in the modified software.
The source code of distributed derivatives must be publicly available