maps the physical device names to instance numbers. it is what is attached to the system and managed by devfsadm (i think, i maybe wrong on this one).
technically, you could manually edit this file but i won't touch it because it can really screw you up. if you ever screw it up, boot to cdrom and copy the /etc/path_to_inst from cd.
anyway i think devfsadm builds the attached devices and maps them in this file. its read only at boot time.

hope this helps a bit.