A new Selector "relation"

It is similar to "/execute on <relation>", the fomat is "relation=<relation>", the "<relation>" in "relation=" is the same as "<relation>" in "/execute on".

For example, set the following comand in a repeating command block and activate it:

/execute as @e[type=arrow] run ride @e[relation=origin,type=skeleton] mount @s

This command will make skeletons ride its own arrow, if the shooter is a pillager/illusioner/piglin/player, the shooter won't ride its arrow, the selector "@e[relation=origin,type=skeleton]" won't find the entity.