How to find "file-release" in OS X El Capitan 10.11.6?
The file(s) /etc/*release contain OS specific informations like distribution name and version mainly in Linux distributions. 

One of the purpose of the file(s) is to help software to detect the distribution type.

A file which contains similar informations doesn't really exist in OS X/macOS but can be mimicked with the following shell script:

#!/bin/sh

OS=`uname -s`
REV=`uname -r`
MACH=`uname -m`
if [ "${OS}" = "Darwin" ]; then
    OIFS="$IFS"
    IFS=$'\n'
    set `sw_vers` &gt; /dev/null
    DIST=`echo $1 | tr "\n" ' ' | sed 's/ProductName:[ ]*//'`
    VERSION=`echo $2 | tr "\n" ' ' | sed 's/ProductVersion:[ ]*//'`
    BUILD=`echo $3 | tr "\n" ' ' | sed 's/BuildVersion:[ ]*//'`
    OSSTR="${OS} ${DIST} ${REV}(SORRY_NO_PSEUDONAME ${BUILD} ${MACH})"
    IFS="$OIFS"
fi
echo ${OSSTR}


Executing the script in my system will yield:

Darwin Mac OS X 13.4.0(SORRY_NO_PSEUDONAME 13F1911 x86_64)




In OS X/macOS the file is not needed because the requirements of a software package are determined by other means:


Info.plist in an installer app (e.g. Install macOS Sierra.app requires LSArchitecturePriority=x86_64 or LSMinimumSystemVersion=10.7)
preflight scripts in an installer packages which use sw_vers or similar commands to determine the OS version.