In a recent beta, context variables were introduced into the game to use in tandem with the arrow operator (->) to query entities marked as what seems to be the built-in subjects (self, other, target, parent, player, holder?, etc). However, these variables do not exist in game. Use of them will throw a log error. Even though, documentation for how these work is very vague.

The beta documentation states that context.variable_name is "(EXPERIMENTAL) Read-only storage provided by the game in certain scenarios", implying that we do not have to initialize and define them for use.

Where is the documentation for how this works?

The 1.16.100.56 beta changelog gives us an example of its use in a custom item in the "Items" subheader, however the variables still are not usable in addons.
https://feedback.minecraft.net/hc/en-us/articles/360049825031-Minecraft-Beta-1-16-100-56-Xbox-One-Windows-10-Android-

This bug report may relate to: MCPE-101702