Thanks, Bob. A couple of other concepts make that option attractive. Once applied to any cell, that workbook will have the custom format available to use in the custom format menu, even if all the cells using it are cleared of formats. Also, it can be applied in several different time-saving ways.
A macro with a dedicated button and/keystroke to apply it to any selected cells (tho that's not as good an option for beginner's).
Or, use the dialog box to apply to one, then use the format painter to apply to subsequent cells. Or, apply once, then use [ctrl]+[y] to apply it to subsequent cells.
Or, select all the cells that require it with [shift] and/or [ctrl], then apply the format either with a button or through the dialog box.
Or, if the rule for when to annotate is simple, you can build criteria into the custom format and apply to the entire column of values. Set it and forget it method.
Of course writing complex criteria in VBA to apply as needed is an option for some, but not most.

However, the cons for using it is the learning curve. It's not any easy system to figure out how to apply criteria beyond the basic positive, negative, zero, non-number syntax And it can be difficult to troubleshoot, modify, or even understand how it's working if someone else inherits the file. I try to make my files user friendly to the next sucker, I mean user, down the line, but sometimes all you can do is implement the complex solution, then add comments. Better still, use a separate tab to make notes. This sometime saves you the embarrassment later when someone asks how something works and you can't remember. That happens a lot.

But now I'm off topic...