According to the base order, we are to follow the most current NFPA standards of the National Electrical Code. 110.16
As of [2011], the relevant section, IMHO, is 110.16 (
emphasis mine):
The marking shall be located so as to be clearly visible to qualified persons before examination, adjustment, servicing, or maintenance of the equipment.
Arguably, that means that the label should be visible before you open the door to examine the breakers, etc.
The section also refers the user, via an informational note, to NFPA 70E, which has additional guidelines.
Now other required panelboard markings, by contrast, are explicitly permitted to be inside the panel, just not too far inside:
408.58 Panelboard Marking. Panelboards shall be durably marked by the manufacturer with the voltage and the current rating and the number of phases for which they are designed and with the manufacturer?s name or trademark in such a manner so as to be visible after installation, without disturbing the interior parts or wiring.
Note that the 408.58 markings must be placed by the manufacturer, and so cannot be on the removable/interchangeable cover, while the arc flash labels are by their nature specific to a particular panelboard in a particular location, and cannot necessarily be supplied by the manufacturer.