The library imported by using zx; statements in .fidl files.
This library is the subset of syscall interface (e.g., its data types) that is modelable in FIDL today. This library is not to be confused with //zircon/vdso, which describes the complement, representing a strict superset of this one (using experimental FIDL). As described there, more of that library will eventually be migrated into this one as it becomes possible to model new constructs.