Why is the restrictive attributive clause incomplete without the meaning of the sentence? Why is there no limit at all?

This question depends on understanding and whether the meaning of the sentence is incomplete. An airplane is a flying machine. An airplane is a flying machine.

This is a restrictive attributive clause. If you remove the anchor, it becomes: an airplane is a machine. An airplane is a machine. Does this sentence still make sense?

Different from the non-restrictive attributive clause, it complements the previous content or is still unfinished. Because it is a supplementary explanation, the original sentence is still complete without it.

For example, he joined the army at 200 1, during which his mother died. If the fixed posts are removed, 200 1 join the army. This sentence has a complete meaning. And "his mother died that year" is a supplement to the previous sentence, right?