[dexter1]

I believe it is not inst which presents you this option, but fx, the disk partitioning tool.

IRIX disks have a volume header which associates partitions with certain volume numbers. In fx you can choose to have a root file system on one partition on the disk, or choose a 'usrroot' configuration where / and /usr are on separate partitions. I forgot the exact volume numbers, but i believe they are 1 for root and 6 for usr. The third choice 'opt' will partition the disk with a single partition with i believe number 7.

The numbers are used for inst to mount the partitions later on in the installation and write it to /etc/fstab in the IRIX filesystem.

In practice for single disk systems nobody is using 'usrroot' anymore, so choosing 'root' is fine. 'opt' is only useful for having a second disk with stuff which is not part of the IRIX OS.