Source

asterisk / README-addons.txt

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===                       Asterisk Add-on Modules                           ===
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    This document pertains to the modules that reside in the addons/
subdirectory of the source tree.  By default, these modules are not compiled
and installed.  If you choose to enable them, you must be aware of what
potential licensing and/or patent implications that has on your usage and
distribution of Asterisk.

    Even though Asterisk is released as open source under the terms of the
GPLv2 (see LICENSE for details), no core functionality in Asterisk has any 
dependencies on libraries that are licensed under the GPL.  One reason a module
may be in the add-ons category is that it may have a GPL dependency.  Since
these dependencies are not compatible with dual licensing of Asterisk, the
dependant modules are set aside to make it clear that they may not be used 
with commercial versions of Asterisk, unless other licensing arrangements are 
made with the copyright holders of those dependencies.

    Another reason that modules may be set aside is that there may be
additional restrictions on the usage of the code imposed by the license or
related patents.  The MySQL and MP3 modules are examples of this.
       
    If you have any questions, contact your lawyer.

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