Why do some network folder icons have darker "tabs"?

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I've used OS 7, 8, 9 for decades now... but not very often in networked situations.

I've set up a home networking server (Netgear ReadyNAS - an older one that supports OS 9/AFP natively). However, when I connect to the server some of my folder icons have a dark "tab" on the folder icon. These are not custom icons -- but the tab on the folder icon is darker blue than the standard folder icon.

I have seen this before when I have used Mac OS networking in the past, but I could never figured out what it meant. Can anyone shed some light on what it indicates?

Thanks for any help you can provide...
-Stephen