Target selectors are used to select certain entities. They accept different arguments to choose which entity to select. Usually, this returns a list of found entities.
Using the limit=n parameter limits the length of the list to n (useful with 1 as n to get a single entity selection). The sort parameter lets you choose which order the list of targets are sorted in.

The sort parameter has 4 options, arbitrary, furthest, nearest, and random. The Minecraft Wiki displays what each of these do:

• sort=nearest — Sort by increasing distance. (Default for @p, @e,^[BE] @a^[BE])
• sort=furthest — Sort by decreasing distance.
• sort=random — Sort randomly. (Default for @r)
• sort=arbitrary — Do not sort. (Default for @e,^[JE] @a^[JE])

nearest, furthest, and random are easy to understand in what they do. The one I am curious about is when the arbitrary choice is selected.

The item returned by a target selector is always an ordered list. What order are they in if they are not sorted?

Possible Theories

Here are some possible theories:

• The list is sorted by UUID.
• The list is sorted by time of creation. (newest->oldest, or oldest->newest)
• The list is sorted by position of entity. Entities are ordered in a scanline fashion, so lowest Z is first, with X from lowest to highest, followed by those on the next lowest Z.