Is LUN to SSCI, fd to file?

Is Logical Unit Number to SSCI, file descriptor to file?

Not really.

A scsi device can present multiple LUNs, each representing a separate
volume. Traditional JBOD disks have a single LUN 0 with the entire disk
on them, but storage arrays can have hundreds of volumes on them, each
using a different Logical Unit Number.
When you want to do I/O to a volume on a scsi device, you specify the
scsi address AND the LUN number. Your operating system will show a disk
for each address/LUN pair, so if you have 4 luns on your scsi device,
your OS will show 4 disks.

Hope that helps,

Arne