When I review a side-by-side diff of the GPLv3 and  the AGPLv3 the only significant change seems to me to be this addition in the section 13 of the AGPLv3:Notwithstanding any other provision of this License, if you modify the Program, your modified version must prominently offer all users interacting with it remotely through a computer network (if your version supports such interaction) an opportunity to receive the Corresponding Source of your version by providing access to the Corresponding Source from a network server at no charge, through some standard or customary means of facilitating copying of software. [...]And later -after the END OF TERMS AND CONDITIONS line- this recommendation on How to Apply These Terms to Your New Programs is different in the AGPLv3 and points back to its Section 13:[...] If your software can interact with users remotely through a computer network, you should also make sure that it provides a way for users to get its source. For example, if your program is a web application, its interface could display a Source link that leads users to an archive of the code. There are many ways you could offer source, and different solutions will be better for different programs; see section 13 for the specific requirements. [...]In this correct to state that the main difference in the terms and conditions is therefore limited to Section 13? or are there any other textual differences that should be considered?If this is correct, would it be correct to consider that the interpretation of the AGPLv3 can be based on any interpretation of the GPLv3 that I could make plus whatever interpretation I can do for this addition to the AGPLv3 in section 13?If 2. and 3. are correct and that I use unmodified code licensed under the AGPLv3, would it be correct to consider that the terms of the GPLv3 and the terms of the AGPLv3 are then essentially the same in this unmodified case? And that therefore any guideline or interpretation of the GPLv3 terms and conditions for this case would apply equally well here?
Of the differences between the GPLv3 and the AGPLv3 texts: what to make of them?
licensing;gpl 3;agpl 3.0
In this correct to state that the main difference in the terms and conditions is therefore limited to Section 13? or are there any other textual differences that should be considered?Yes, your analysis of the text is correct. The FSF says of the AGPL (emphasis mine):The GNU Affero General Public License is a modified version of the ordinary GNU GPL version 3. It has one added requirement: if you run a modified program on a server and let other users communicate with it there, your server must also allow them to download the source code corresponding to the modified version running there.Given the fact that Section 13 is the only difference, and the FSF summarize the AGPL as having only one difference from the GPL, it is correct to understand the AGPL as otherwise identical to the GPL, outside of the addition of section 13.If this is correct, would it be correct to consider that the interpretation of the AGPLv3 can be based on any interpretation of the GPLv3 that I could make plus whatever interpretation I can do for this addition to the AGPLv3 in section 13?Yes, that sounds basically correct. In fact, the FSF's GPL compatibility matrix includes the note:Each place that the matrix states GPLv3, the same statement about compatibility is true for AGPLv3 as well.So we can certainly know that compatibility within the xGPL family is identical for GPLv3 and AGPLv3. As for its compatibility with other licenses, the only case I can think they would differ is: if there were some hypothetical license that wasalmost GPLv3-compatible, exceptit imposes a requirement identical the requirement in AGPLv3 section 13then such a license would be GPL-incompatible but I think it would be AGPL compatible, since such a requirement would be additional to the GPL's terms, but not additional to the AGPL's terms. As far as I know, no license like that actually exists.If 2. and 3. are correct and that I use unmodified code licensed under the AGPLv3, would it be correct to consider that the terms of the GPLv3 and the terms of the AGPLv3 are then essentially the same in this unmodified case? And that therefore any guideline or interpretation of the GPLv3 terms and conditions for this case would apply equally well here?The first paragraph in Section 13 begins with a conditional: if you modify the Program... If you don't modify the program, then that paragraph is not relevant to your use of the software. I have written to the FSF licensing team and had this reading confirmed.The only case that would be different (as  far as this layman can tell) would be cases concerning downstream recipients who do modify the software. So, if you aren't a downstream recipient who modifies the software, then your obgligations under the AGPLv3 would be the same as under the GPLv3.