Note: This guide is for PetaLinux version 2019.2 and older. For PetaLinux version 2020.1 and newer, see this newer post.

This post shows you how to figure out which version of Yocto is being used in a particular version of PetaLinux Tools (PetaLinux Tools 2018.2 is using Rocko 2.4.1).
Versions Used
PetaLinux Tools 2018.2
Steps
1. Type cd /opt/pkg/petalinux
2. Type find . -name poky.conf
You'll see the following output:
Note: this implies that PetaLinux Tools 2018.2 ships with 4 copies of Yocto.
3. Type find . -name poky.conf -print0 | xargs --null grep -HEi 'DISTRO_CODENAME =|DISTRO_VERSION =' to list the DISTRO_VERSION and DISTRO_CODENAME for all 4 of the Yocto copies shipped with PetaLinux Tools
You'll see the following output:
This means that PetaLinux Tools 2018.2 is using Rocko 2.4.1.
With this, the specific documentation for the release can be listed at [link] by selecting YP CORE - ROCKO 2.4.1.
Examples
Yocto Project Reference Manual for 2.4.1 at [link]
Yocto Project Mega-Manual for 2.4.1 at [link]
Yocto Project Quick Start for 2.4.1 at [link]
References