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This is my understanding about how software licenses work in most jurisdictions — I add this, because it might help to clearify things:

1. When I attach a license to a program, the license governs my copyright.


2. You can only change the license in case you are compliant with the current license terms or all the copyright-holders agree to change it.


3. Because the license governs my copyright it also governs how license updates work.
   
   
   
   
   • In case nothing is specified in the license on how license updates work — copyright law does apply.
   
   
   • In case it is specified how license updates work the license entirely governs how license updates work. For example oversimplified when my license says you can do whatever you want with that code even attach another license, then what is stated in there counts and copyright law is superseded by the license.
   
   
   
   

Quotation from the GPLv3:

14. Revised Versions of this License.

The Free Software Foundation may publish revised and/or new versions
of the GNU General Public License from time to time. Such new versions
will be similar in spirit to the present version, but may differ in
detail to address new problems or concerns.

Each version is given a distinguishing version number. If the Program
specifies that a certain numbered version of the GNU General Public
License “or any later version” applies to it, you have the option of
following the terms and conditions either of that numbered version or
of any later version published by the Free Software Foundation. If the
Program does not specify a version number of the GNU General Public
License, you may choose any version ever published by the Free
Software Foundation.

If the Program specifies that a proxy can decide which future versions
of the GNU General Public License can be used, that proxy's public
statement of acceptance of a version permanently authorizes you to
choose that version for the Program.

Later license versions may give you additional or different
permissions. However, no additional obligations are imposed on any
author or copyright holder as a result of your choosing to follow a
later version.

Question:

1. Does that mean that when you specify GPLv3 or any later version that anybody who receives the code may add a later version that may not be necessarily compliant with the current version? I mean since from my current understanding the license governs the copyright (see point 3 above) and the license is unclear at that point, I ask my self this question.


2. Does that mean that when you specify an explicit GPL version number without having the "or any later version" that implicitly anybody who receives the code cannot add another GPL version that is not compliant with the current version?

1. Does that mean that when you specify GPLv3 or any later version that anybody who receives the code may add a later version that may not be necessarily compliant with the current version?

Yes, if you indicate your license as "GPL version X or any later version" then recipients may operate under the requirements of GPL version X or any higher-numbered version, despite the fact that versions of the GPL are incompatible with one another (and future versions may be as well). This is explicit in the license text you quote:

If the Program specifies that a certain numbered version of the GNU General Public License “or any later version” applies to it, you have the option of following the terms and conditions either of that numbered version or of any later version published by the Free Software Foundation.

It is true, generally, that a downstream recipient may not change the license terms. However, the licensor has explicitly allows the licensee to remain in compliance with the grant when following different terms, per the GPL's language "you have the option of following the terms and conditions... of any later version". To be clear, this isn't a change of the licensor's terms; rather, it is an option within the licensor's original terms that always existed.

2. Does that mean that when you specify an explicit GPL version number without having the "or any later version" that implicitly anybody who receives the code cannot add another GPL version that is not compliant with the current version?

Indeed, the quoted section above is a conditional statement. If the licensor chooses not to meet the condition when licensing the software (i.e., they choose not to "specif[y] that a certain numbered version of the GNU General Public License “or any later version” applies to it") then the associated permissions under that conditional are not in effect.

1. If you specify "GNU GPL v3" without "or later" then that is the only license other people can distribute the code under. That's why Linux is still licensed under version 2.




2. If you specify "GNU GPL v3 or later" then you are multi-licensing your code. The two licenses do not have to be compatible, and based on previous license updates, will usually be at most one-way compatible. Only do this if you trust the organisation not to publish a new license which goes against the spirit of the current license. You can always republish it if they release a new version in the future.