If this refers to a specific work, it is most likely a "reincarnation" or "transmigration" story, where characters are often given numerical designations in futuristic or experimental settings. Common themes in these romantic storylines include: Common Relationship Archetypes

A girl is transported into a historical romance book? : r/RomanceBooks

While there is no widely known single book or series titled the phrase likely refers to specific character codes or fan-favorite tropes within the niche of online webtoons, manhwa, or romance novels that feature digital identification or experiment-based numbering.