What does X-Accel-Limit-Rate really do in NginX?

These are the docs for X-Accel-Limit-Rate:

Sets the rate limit for this single request. Off means unlimited.

Not much there. Most of the examples (I've found only two or three) I've seen set the value of X-Accel-Limit-Rate to 1024. This is obviously 1024 bytes, but per what? Or is that a tota开发者_如何学运维l of some sort?

Without knowing what the value means it's difficult to know what it's really doing.

Apparently it is bytes per second. Source