Logical unit number / sub interface

Hi,

Sorry to sound absolutely green.

Just started on Juniper platform and is not really used to the concept of logical unit number of the physical interface.

Been seeing these below output in my switch config

em2 up up
em2.32768 up up inet 192.168.1.3/24

vme up up
vme.0 up up inet 192.168.0.203/24

q1) How does one decide to use a logical unit number of 0 ? and a logical unit number of 32768 ? - if the purpose is to just give the interface an IP address.

I also see some logical unit number of 16385, in the loopback interface

lo0 up up
lo0.16385 up up inet

Regards,

Alan