8sync is released under the GNU LGPL (Lesser General Public License), version 3 or later, as published by the Free Software Foundation. The short version of this is that if you distribute a modifications to 8sync, whether alone or in some larger combination, must release the corresponding source code. A program which uses this library, if distributed without source code, must also allow relinking with a modified version of this library. In general, it is best to contribute them back to 8sync under the same terms; we’d appreciate any enhancements or fixes to be contributed upstream to 8sync itself. (This is an intentional oversimplification for brevity, please read the LGPL for the precise terms.)
This usage of the LGPL helps us ensure that 8sync and derivatives of 8sync as a library will remain free. Though it is not a requirement, we request you use 8sync to build free software rather than use it to contribute to the growing world of proprietary software.
The choice of the LGPL for 8sync was a strategic one. This is not a general recommendation to use the LGPL instead of the GPL for all libraries. In general, we encourage stronger copyleft. (For more thinking on this position, see Why you shouldn’t use the Lesser GPL for your next library.)
In particular, if you are building a library or application that uses 8sync in some useful way, consider releasing your program under the GNU GPL or GNU AGPL! In a world where more and more software is locked down, where software is used to restrict users, we could use every chance we can get to provide protections so that software which is free remains free, and encourages even more software freedom to be built upon it.
So to answer the question, “Can I build a proprietary program on top of 8sync?” our response is “Yes, but please don’t. Choose to release your software under a freedom-respecting license. And help us turn the tide towards greater software freedom... consider a strong copyleft license!”