The option is only needed on certain models. This option was created in Junos 10 when the new feature of an alternate boot partition was added to Junos. this allows the boot recovery of the device when the primary partition is corrupted during a power failure event.
The partition option creates the second partiion where none existed before. Any Junos device manufactured and release after this addition does not need the option.
Note: Partition install is supported on the default media on SRX100, SRX210, and SRX240 devices
(internal NAND flash) and on SRX650 devices (internal CF card).
Partition install is not supported on the alternate media on SRX100, SRX210, and SRX240 devices
(USB storage key) or on SRX650 devices (external CF card or USB storage key).