Magic ID was originally a literal "mysterious ID that cannot be easily deciphered", but on the web, it is a code that can only be deciphered by a special group of people...
"Magic ID" is commonly seen in forums, online communities, and similar platforms, especially when experienced users are sharing NSFW content. For example, when a user posts NSFW content, other users might reply with "OP, please share the Magic ID".
Generally speaking, Japanese works' production companies assign unique product codes consisting of letters and numbers to their own products for identification, sorting, and retrieval purposes. These seemingly simple yet crucial codes, known as "番号" (pronounced "bangou"), are completely unknown "Magic IDs" to the uninitiated. However, for seasoned connoisseurs, they can quickly discern the production company, style, and actors associated with a particular code and promptly search for suitable resources on certain platforms to download and enjoy.