The purpose of SID.

[dE <de.techno@xxxxxxxxx>]

As I read in the SELinux docs, each subject and object is assigned a 
unique SID; when using the selinux libraries, or using the SELinux 
kernel API the programs are expected to request the security server 
decisions for a particular subject and object by passing the subject and 
object's SID to the security server.

Question is -- is SID created when an SELinux enabled kernel boots or 
just when a SELinux enabled program requests an SID for a subject/object 
from the kernel?

Also can I see a process's and file's SID via some program?