alt Hacker News> If the subject can utter the action, then it can perform it.
This sounds like another layer of weird terminology that doesn't mean anything for someone who is not familiar with whatever capability system you're thinking of.
Say I am a user who can see a particular directory on a shared setup. I try to upload a file in this directory, using the same method that worked on another directory. The question of AuthZ is: will I be allowed to do it or not? In the plain sense of the words, I can absolutely "utter the action", I have all of the "verbs" (upload) and "nouns" (the file, the destination path). Still, I should not be allowed to perform the action if I was only given read-only access here.
Now sure, you can say that "upload to dirA" is a different verb than "upload to dirB". But this is just confusing terminology, it doesn't enlighten anything.
You seem to understand it just fine.
Your accessor, dirB, should not contain the “upload files” verb, while your dirA accessor (noun) should.
My favorite example is the home directory and the file picker. Why should a program have access to all your files by default then politely ask you which file it should read/write to? It would make more sense if the file picker was something the operating system ran when a program wants to edit a file, and what came back to the program after you selected was the accessor for that file (with read and/or write verbs).
So the program only have access to those files you have it access to. It cannot even ask the question to open another file, because it only has opaque accessors to those files it has been given.