Pass through device support in tgt

Hello,

I am trying to understand the passthrough device support of tgt.  For
a regular block device, to create a LU I use the following command ( I
explicitly stated the default values below):

tgtadm --mode logicalunit --op new --tid 1 --lun 1 --device-type disk
--bstype rdwr --backing-store  /dev/sdb

The above command takes the block device "/dev/sdb" and exports it as
LU 1 to the initiator.
- What does it mean the tgt "creates a device of type disk"  Creates
where, internal to tgt?
- What is the implication of that? Is it how the SCSI commands coming
over iSCSI is treated/handled  (with respect to caching, buffering,
read ahead, etct)? Or something else?

The following example is cited in  the man pages for a passthrough:

tgtadm --lld iscsi --op new --mode logicalunit --tid 1 --lun 1
--bstype=sg --device-type=pt --backing-store=/dev/sg4

- How does the "bstype" affect the handling of the SCSI commands?
Does this affect how the IO to the actual device is done (manipulated
before sending to the actual device)?
- What does it mean for the backend storage type to be sg?

Really appreciate any help / pointers.

Regards,
Ahmed.