TL;DR: Never use the custom flagging reason unless the reason for flagging something isn't covered by the options above the custom one.
I made this topic because I keep seeing people misuse the custom flag reason we have. The custom flag reason is there to let us know about an important reason for the deletion of the image, one that is not covered by the other reasons.
Seeing how there is a reason that literally says "inferior/duplicate version" the custom flag reason should never be used to just tell us "I uploaded a better version", "I accidentally uploaded the small version", or similar. All of these flags should simply be done under the inferior version flag.
Also, if anybody told you to always use the custom flagging reason if you can they are wrong. Don't do that.
If anybody told you "but using the custom flag reason doesn't count against your deleted posts counter / upload limit" they are wrong.
Updated by SnowWolf