"Nameplate" stickers were commonly applied to the side of the breakers, this could be one of those.
Being familiar witrh the older Westinghouse breakers which this one looks similat to the old LA3400, the labels were placed either to the right and/or left side of the toggle with the modification sticker added. I have found that some devices don't have enough area of the face to place the lable and then if become necessary to attach them to hte side. In doing so it makes then very difficult to place yourself in a position with enough light and move your head in a position to read those labels.
It's interesting that the breaker manufacturer having so much space available on the face of the breaker chose the side for the labels which makes it somewhat difficult for the electricians.
Or data centers. I seem to recall that there is an NFPA or other standard that requires that, but could be faulty memory and was some other standard practice, like IBM used to set standards for their data center constructions that everybody just adopted back-in-the-days.
EPO. Emergency Power Off
Required only if you choose to use the leniencys found in Article 645.