That is where BMW compares the N55B30T0 under the hood of the M2 to the S55 motor used in the F80 M3 and F82 M4 models as well as the N54 which is well known around these parts.
It is interesting to note that the engines are all the same basic thing. They have the same firing order, same compression ratio, same bore x stroke, same number of valves per cylinder, same fuel injection, etc. It's all the same engine geometry and family.
Despite BMW detailing in the document how the N55B30T0 shares the S55 pistons and many other parts they state the engine is a revised N55 from the F22 M235i. Considering it is a closed deck block design with S55 pieces it seems to be more of an S55 with the N55 turbo setup.
It remains puzzling why BMW does not give the M2's motor an 'S' designation considering it shares so many S55 parts and is closer to the S55 design than the N55 design. Not to mention that it goes under the hood of an M car. Maybe BMW did not want to make it seem like the M2 is a cheaper M4? It's an odd decision when you read the details.