From what I recall, "Restricted" was a security classification applied by the Brits and no one else. Other nations tend to have "Confidential" as their lowest classification. For some reason, in days of yore, other nations were misguided enough to think that Catering Manuals and Drill Manuals need not be classified.
I remember referring to UK manuals that were "Restricted" but not finding the info I was after and having to go to the NATO source documents for more info or clarification and finding that the NATO docs were unclassified.
So we get info from NATO or others nation's publications that are unclassified and readily available to every Tomaso, Riccardo or Henri and incorporate this info in one of our publications and give it a security classification. You can buy a US FM or TM off the interweb and leave it lying around no problem, however, if you leave a Brit publication that was copied from those US publications around and one of your betters or one of the rubber heeled gang comes across it, you could say goodbye to your LS&GC.