Regulations object
Regulations is the name OpenRTB 2.2 gave to the object at BidRequest.regs, which carries the legal and regulatory signals for a request. In 2.2 it held only the COPPA flag and an extension object. From 2.3 the same object is called Regs.
How it is used
The rename changed nothing on the wire: the JSON key was regs in 2.2 and stayed regs afterward. Later versions added GDPR, US Privacy, and GPP fields to the renamed object, so a 2.2 payload that carries any of those signals puts them under regs.ext.
The Privacy signals page covers it in more depth.
Fields
All objects
The field reference index lists every OpenRTB 2.x object.