Nellisir said:
I am not a lawyer, but it's typically understood that there are three kinds of content - open content, product identity, and closed content. If it's not defined as open, it's usually considered to be closed.
Is there really a difference between PI and "closed" content? You can't freely re-use either without a separate agreement (aka license) with the creator.
If I read the original poster's question correctly, restated how I interpreted it, they asked:
Does [that clause] in the OGL legally require ALL content, which includes any amount of OGL-licensed material, to default to OGC classification if it is not specifically marked PI?
From what I can tell, no it doesn't, specifically allow a content creator to assume "all content not explicitly marked PI is OGC." Many OGL books have OGC/PI statements that seem clear enough at first read, but if you carefully re-read them, and try to apply it to various pieces of the material, you find questions, as is evident on forums worldwide.
With copyright law still applying "behind" the OGL, any ambiguous content, not explicitly marked or declared OGC, is still "copyright-but-not-marked-OGC", which I interpret as being covered under the definition of PI. If there is any question or shadow-of-a-doubt at all whether an item is OGC, its best to assume its not, IMO.
Also, be aware that nothing in the OGL prevents the original creator of the work from modifying the non-explicitly marked sections later. If, for example, the publisher posts a "clarification" on their website, some legal venues may incorporate that clarification into the legal proceedings as representing the original "intent" of the content creator. If there was any ambiguity at all in how the content was marked, clarifications like that generally swing the outcome of the legal proceeding in favor the original creator.
This is exactly what's happened with WotC's website "clarfications" around OGL and software. The OGL and Software FAQ should be considered "de facto" inclusions to the OGL itself, because a court would generally respect the "intent" those additional documents represent.
This question aims at the heart of one of the more annoying problems with OGL/OGC in general: Ambiguity.
Many OGL publications are vague, ambiguous, or in a few cases even intentionally deceptive, about what is PI and what isn't... in other words, there are a million questions in forums around the world about what you can re-use/re-mix and what you can't, which is sad since that is exactly what the OGL was supposed to prevent in the first place.
Questions like this also show the chilling effect that a few third-parties have created from deceptively labeling content OGC, then intimidating re-users to ask permissions or otherwise behave as if the content was PI.
And, it shows the chilling effect that the OGL ambiguities cause as content developers wonder if WotC or some third-party is going to somehow ambush, mousetrap, or otherwise spring an unpleasant surprise on them in the form of legal action after publication, because the legalistic aspects of the material are so prominent.
Everything is risk, so just do what seems "right" and "proper", and just go for it.
Get a lawyer's help if you are worried.
If you are worried a lot about the risk, then maybe its better to avoid the OGL (and all content licensed with it) and stick with something a bit clearer like FSF's GPL or Creative Commons licenses. Just compare the amount of "can I reuse this GPL code or CC-licensed material" questions to the amount of "can I use this OGC" questions around, and you will see what I mean about clarity.
If you must use OGC, then cover your bases as much as possible and try to limit what you use to only content pieces that are marked explicitly as OGC. Also, be sure you can demonstrate how you determined or knew that block was OGC, for each piece you reuse. Each publisher can choose whatever means they like to "clearly indicate", but some means are "clearer" than others. If the means is to use two different but similar looking fonts, it can be easy to mistake them. If its in a chapter that the publisher declares the entire chapter OGC with no exceptions, then that's a bit easier.
Your best bet for keeping the OGC and PI lines clear and distinct is to copy out all of the explicitly defined OGC from a supplement (and the reference that explictly defines it) into an entirely separate set of references, then work only from those. Lock up the books or PDFs which are not 100% OGC so that you can't accidentally re-use any PI or maybe-PI from them, going forward.