Omni Web Flags vs Tags

I am new to OmniFocus iOS and OmniFocus for the Web. Can someone help me understand - or point me to a resource/video - the difference between tags and flags in the web version. I’ve read where some use flags to identify important, non-dated items. But what is confusing is why the lists of flags and tags are identical? Also, when I add, delete, or rename a flag the same change is made to tags. Thanks in advance.