They used to call it Section 8.
It covers a lot of psychological, emotional, sexual, etc grounds for dismissal from the U.S. army.
In the 1960s - when many young men wanting to avoid the Vietnam draft had no hope of using college draft deferrals, religious vocation, fatherhood, etc but had read Catch 22 - this was a popular way of making your sergeant throw you out of the army.
Here's the piece of script from the Sinatra movie, Suddenly, where we all learned about it:
You know, Baron, you got the yard look.
You look yard bird to me.
I'm tellin' you, sheriff. Turn it off.
Big shot with the yard birds,
eh? Come on now, tell me.
Where were you court-martialled?
What was it for? Rape?
No, not rape. Killing,
that's what you like, killing.
Maybe shooting down
Sit down and shut up.
Ah, but by golly you got away with it.
So... I got it. Section eight.
You went out on a section eight.
Psycho in charge of killing, eh?
You know, I knew guys like you.
Killing was sweet.
Rather kill a man than love a girl.
A real kick. A thrill with the guts.
Jimi Hendrix used it. Maybe Jim Morrison too. I often wondered how the members of The Eagles dodged the draft.
It was also a way of ridding the army of psychopaths (guys who loved killing so much it had a demoralising effect on their fellow soldiers), guys who just wouldn't take orders (Sinatra and Kerouac got thrown out on this ground) and maybe guys whose battle shock in the Pacific War made them obviously unusable for further fighting.
The U.S. army in recent years replaced this provision with something more specific and cognizant of modern sociology and psychiatry.