For the check-box "Restrict where this project can be run," there is nothing in the help that mentions what kind of expressions you can write.
From trial and error, I deduced that "a || b" means "use either a or b as build nodes." I expected a comma, like other fields (eg. projects that can access archives, projects to run after this build, etc.) I'm sure there are other valid expressions, such as using "*".
Can you please update the help for this field to include some examples? I ended up looking at the source on GitHub and guessed this from the use of Antlr.