Security Theory LLC RFC 8949 defines CBOR, a standard for serializing data types such as integers, strings, and arrays into encoded bytes. CBOR serialization is flexible, allowing data types to be encoded in multiple ways to accommodate deployment in constrained environments. This document normatively defines one particular serialization, called "preferred- plus serialization," that is suitable for the majority of CBOR-based protocols. Protocol designers and implementers who choose it need not understand or specify serialization details themselves. This document also normatively defines a deterministic serialization. These serializations are largely compatible with those widely implemented by the CBOR community. This document updates RFC 8949 with a new rule that limits how new tag definitions can affect the CBOR data model. This document provides clarifications to RFC 8949 regarding bignums and floating-point NaN handling, along with general background information on serialization, determinism, and CBOR byte-string wrapping.