"Request system halt" on an SRX device is a command used to shut down the operating system safely. It ensures all running processes are terminated properly, filesystems are unmounted, and the device is prepared to be powered off.
This command is handy before physically powering down or rebooting the device to avoid any potential data corruption.

Syntax for SRX Series devices below:

request system halt
<at time>a
<in minutes>
<media (compact-flash | disk | usb)>
<message "text">

On SRX1500/4100/4200/4600 "request system halt" will not work. On these devices, you need to use '‘request system power-off”

The difference between "Request system halt" and "request system power-off" is that the "request system power-off" command safely shuts down the operating system, but it goes a step further by physically powering off the device. The system is fully turned off, and no manual intervention is needed to remove power.
In short, `halt` stops the OS, while `power-off` stops the OS and cuts power.

Note:- "request system power-off” will work on all the devices. After this command, you need to manually power on the device, whereas while using the command  "request system halt" you can reboot the device by pressing any key.

Sample Output below:-

root@srx345> request system halt

warning: This command will not halt the other routing-engine. If planning to switch off power, use the both-routing-engines

option.
Halt the system ? [yes,no] (no) yes

*** FINAL System shutdown message from root@srx345 *** System going down IMMEDIATELY

Shutdown NOW! [pid 2193]