Where can I find guidelines on when to use the red octagon with exclamation mark icon in Mac OS?

Question

This icon is shown under Mac OS when a disk is unmounted without being ejected first.

It is sometimes also used by other applications. Can someone point me to a place where the acceptable uses for this icon are explained, if this exists? I couldn't find it in the HIG.

Answer 1

From that one example, and from these:

It would appear that the octagonal icon is used when it is reporting an event that has occurred, and it is too late to do anything about it. This would be different from a triangular alert icon, which appears when you are being asked to do something with some caution.

Answer 2

This icon seems to be largely a hold-over from the classic MacOS days (before Mac OS X). Apple is now recommending that you only ever use either the application icon or the yellow caution icon (called, interestingly, the NSCriticalAlertStyle). The latter is, apparently, supposed to be used in the same situations that the previous "stop" icon was used (namely in situations of possible or probable data loss):

This type of badged alert is appropriate only if the user is performing a task that might result in the inadvertent and unexpected destruction of data.

In classic MacOS, alerts never used the application icon; instead they used one of the following icons (these icons taken from The Interface Mafia):

A clue to why the icon was deemphasised (or even removed) in OS X is the fact that the "stop" icon was apparently supposed to be replaced in other localisations:

The stop icon is replaced by another icon in some countries where an open palm has offensize meanings.

Since Carbon applications were the main way for classic Mac apps to be ported to Mac OS X, it's likely that the icon was produced (without the potentially-offensive gesture) to enable apps to run cross-platform.

It seems fairly clear to me that Apple would prefer you didn't use it going forward.