From the sidebar:

Submission rules:
2. Please try to include the series name in your title. Characters not from any series are tagged as [Original]. Original content made by you is tagged as [OC]

As stated above in rule 2, the bracket tag is for signifying if the work is from a series (for example [Genshin Impact]) or if it's an original work not from a series. Any work that's submitted by its own creator are meant to be labeled with [OC].

In this context, AI art that's not based on an existing series, as set by the rules set of this subreddit, shall use the [Original] tag.