Is this an illegal 2 unit or what is a “zoned in law”

[Anthony Mei]

https://www.redfin.com/IL/Chic...

This house is advertised as a 2 unit but it’s zoned RS3 it seems based on database. The description says “zoned in law” for the top floor unit. What does that even mean? Is that a thing for Chicago or is it just a made up term

Is whoever renovated this just not following the city and they turned a single family into a multi unit when they shouldn’t have? Or is this house somehow actually a legal 2 unit?