If I want to make D&D content do I use the OGL? What if I want to use open content that is from outside the SRD 5.1? Should be available, yes?
What if I want to use the CC license? Can I pull in open content released under the OGL from outside of the SRD 5.1?
So, it's important to bear in mind there are two parts to a license. It's also
important to bear in mind that I am not a legal expert, just someone thats worked with licensing for 23+ years and picked up stuff along the way. Oh, and I'll also point out that once ORC material becomes available, you can substitute ORC for "OGL 1.0(a)" in any of the below explainations.
There's the "export" end of the license, which grants permission for the content under that license, and the "import" end that you use in order to get that permission.
Traditionally, a license would grant the 2nd party the rights to content produced by the 1st party, and that's it. That's how licenses like CC-BY and the proposed OGL 1.2 work.
Then we have so-called "viral" licenses, which not only do the above, but also pass the right to use content on to the next
user by requiring that any work produced using the "import" end also include a new copy of the "export" end.. That's things like the real OGL 1.0(a), and the CC-BY-SA license.
SRD 5.1 is available to use under both CC-BY (specifically, CC-BY-4.0) and OGL 1.0(a)
If you use SRD 5.1 under CC-BY, you get substantally more rights, because there are no exclusionary clauses. You can use anything in that material, in return for an attribution in your work. This will not confer any rights on users of your work to anything, although they can always use SRD 5.1 themselves by using their own CC-BY license from WotC. It does not make your
work CC-BY as well, it confers no reuse rights on others. If you can find any other material out there that is available under CC-BY, you can also use that by adding the additional attribution. An important note here is that you do not
reproduce the CC-BY license in your work, you only
put the attribution in. Otherwise you're creating a brand new CC-BY license others can use too (and see below as to why you probably don't want to, and potentially cannot legally, do that)
If you use SRD 5.1 under OGL 1.0(a), you cannot use anything declared "Product Identity" in the SRD. As OGL 1.0(a) is a viral license, you also have to use OGL 1.0(a) for your own work, but it is up to you how much of your work you declare "Open Game Content" (you can technically declare nothing Open Game Content other than material that came from the SRD, but that will get frowned upon because you're happily using other peoples content without donating any of your own.) Every source you use, you must copy the entire
Section 15 from that source into your work (even if you don't think you used anything from the other works mentioned in there, because chances are you have without realizing it and because thats a requirement of the license.)
Finally, there's the option of using both. You can use SRD 5.1 under CC-BY through placing the correct attribution, in addition
to using OGL 1.0(a) to pull in work from other sources that are only licensed under that. Once you've done that, you declare open game content in your
work, which others can then use through OGL 1.0(a).
You'll note that with CC-BY not being a viral license, none of the above examples included you making your
work available under CC-BY. If you want to do that, things get a little more complicated. Firstly, you have to own copyright to everything in that work in order to CC-BY it (or have permission from all other copyright owners to make their work available under CC-BY - if the contribution itself
is CC-BY, such as SRD 5.1, then the CC FAQ states you can do it, and how to). I'm assuming there may be artwork in that work you also do not own the copyright to, but only a license to use. If you're making this content commercially, you've also just given every buyer the right to send copies of the entire thing to anyone they want (but hey, if you want
that, and have determined that you legally can
do that, go for it
Assuming a commercial work including things such as artwork you don't own the copyright to and some text that you don't
want to give away for reuse, at this point you need to do exactly what WotC did in creating the SRD from the PHB/MM/DMG - you create a seperate document that only
contains the parts of the work you own the copyright to (or have permission to include in a CC-BY licensed offer, such as SRD 5.1 and other CC-BY offerings ) and wish others to have permission to reuse (with attribution), and attach a CC-BY license to that
document. For example, you might remove anything related to a fictional setting you've created and just leave the monsters, character classes, spells, or whatever. What you leave in there is totally up to you
You can then put that shiny CC-BY-licensed document on your website, bundle it in with your book on DTRPG, or distribute by any other manner you see fit.
(edited with corrections about using CC-BY on your own work if you included CC-BY material)