explanation of the designation of nightly builds.

It's a tag to let create the exact same build again if you'd like. It's a tag from github. For example you can follow this link to see the exact patches that went into building that exact image:

GitHub - LibreELEC/LibreELEC.tv at 1fcfe6aa3893e400bd1828d8e18fdc842980c795

This is the shortened githash of the master branch the build was created from - you can look it up eg with Merge pull request #4556 from CvH/10.0/espeak · LibreELEC/LibreELEC.tv@1fcfe6a · GitHub

so long,

Hias