The compatibility license has a number of great real-world analogies.
You can claim compatibility with someone else's product with no special permission needed. Look at all those iPhone cases and chargers and things which claim compatibility with iPhones; those aren't licensed products. Same with things designed for models of car, cartridges for printers, and so on.
So simply claiming compatibility is not trademark infringement by itself.
The OGL isn't about trademarks, though. It's about copyright. Or, more to the point, it's an agreement whereby you completely circumvent copyright issues because you have permission to use certain text. You can happily use the description of a spell or a monster's statblock verbatim, and you're doing so with the property owner's blessing.
In exchange for that, you agree to a few things, none of which are particularly onerous. The most important thing you agree to is to not indicate compatibility in a way for which you don't have specific permission. In other words, you agree not to put "D&D" on the cover.
Back then, you had permission to use the d20 logo instead; not any more though. That logo permission has been rescinded.
So you can use the OGL, which gives you loads of free text you can copy as much as you want, but can't use any trademarks without permission.
Or you can indicate compatibility with trademarks without permission, but then you can't use the OGL to get access to all that free text. You have to rewrite every word yourself.
You can claim compatibility with someone else's product with no special permission needed. Look at all those iPhone cases and chargers and things which claim compatibility with iPhones; those aren't licensed products. Same with things designed for models of car, cartridges for printers, and so on.
So simply claiming compatibility is not trademark infringement by itself.
The OGL isn't about trademarks, though. It's about copyright. Or, more to the point, it's an agreement whereby you completely circumvent copyright issues because you have permission to use certain text. You can happily use the description of a spell or a monster's statblock verbatim, and you're doing so with the property owner's blessing.
In exchange for that, you agree to a few things, none of which are particularly onerous. The most important thing you agree to is to not indicate compatibility in a way for which you don't have specific permission. In other words, you agree not to put "D&D" on the cover.
Back then, you had permission to use the d20 logo instead; not any more though. That logo permission has been rescinded.
So you can use the OGL, which gives you loads of free text you can copy as much as you want, but can't use any trademarks without permission.
Or you can indicate compatibility with trademarks without permission, but then you can't use the OGL to get access to all that free text. You have to rewrite every word yourself.