Meaning/usage of "送りを指す"?


This is an extract from a piece of fiction:

> 基本的に『除隊』は『レベル・ゼロ』という特殊牢送りを指す『The Tower』において、彼が職を辞すには正式に引退する必要があったのだ。

I am having a bit of trouble understanding what "送りを指す" means in this context
and it's not a phrase I've come across yet in my studies. How is it being used
here?

From my attempt at a translation, I think it would be something like:

> Basically, as for a "discharge from military service" it was necessary to
> officially retire in order to quite his job at "The Tower" [送りを指す] which is
> special prison called "Level Zero".

So what could the 送りを指す mean?

To me the best I can think is:

> Selected to be sent

Which doesn't seem to fit in the sentence at all. I think there's a
possibility it's some sort of set phrase that can't be translated too
literally since I've seen a lot of that in Japanese. But if someone could
clarify that would be awesome, I'm really curious about what the extract is
actually saying!




> 基本的に『除隊』は『レベル・ゼロ』という特殊牢送りを指す『The Tower』において、彼が職を辞すには正式に引退する必要があったのだ。

See the other answer for what 特殊牢送り means. Here, 送り itself means the action of
sending (see [this answer](https://japanese.stackexchange.com/a/32311/5010)).
牢屋送り is a relatively common set phrase meaning _imprisonment_. Everything
before "The Tower" is a relative clause that modifies "The Tower". So the
whole sentence means:

> In _The Tower_ , where "discharge" basically meant being cast into a special
> prison called _Level Zero_ , he had to officially retire in order to quit
> his job.

