I thought it was easy to find once you had the documents on screen. My apologies. I've probably spent too much time reading JAR/CAA/FAA Volapük, I'm not even registering the incomprehensibility and difficulty of navigating it anymore...
Here are the exact references:
JAR-OPS 1.620
AC 120-27E §201-205
If you want to understand standard weights, a more thorough read is recommended. AC 120-27E is actually a rather good bundle of Volapük! If you want a challenge, get hold of one of the earlier editions. The FAA did a good job updating it.