It's the vertical line that separates the different names of a character, so, between "Shouta" and "Eraserhead" in this example: Aizawa Shouta | Eraserhead

You can decide which name you'd like to use, so nominate tags as e.g. "Aizawa Shouta & Shinsou Hitoshi (BNHA)" or as "Eraserhead & Shinsou Hitoshi (BNHA)". Then just hit enter even though the AO3 dropdown menu might offer the regular tag as an option.