This is xnu-8019. See this file in:
21.3.0

# The first line of this file contains the master version number for the kernel.
# All other instances of the kernel version in xnu are derived from this file.
#
# The format of the version number must conform to the version resource format
# as described in TN1132: http://developer.apple.com/technotes/tn/tn1132.html
#
# In particular, the string is formatted as: J[.N[.R[S[L]]]], where:
#  J represents the kernel major version number (integer)
#  N represents the kernel minor version number (integer)
#  R represents the kernel revision number (integer)
#  S represents the kernel build stage (one of "d", "a", "b", or "r")
#  L represents the kernel pre-release level (integer)
#
# The correct way to make use of the kernel version within kernel code or a
# kext is to include libkern/verison.h.  version.h contains defines that can
# be used for build-time version logic and prototypes for variables that can
# be used for run-time version logic.