i’m not sure I understand your question. if your “What scsi..” mean Why the answer is that the SCSI protocole was built tike that. If the “what scsi…” mean What quind of device can be connect to scsi. the answer is anny kind of scsi device as long as you close the line. mean that the last device of the chain as a terminator an you don’t have more than 15 divice plus the card.
Wide SCSI and better allow up to 16 devices on the same cable although you are actually restricted to 15 devices as one is the host itself.
as to improving things with SCSI there is a paramaeter called skew which means that parallel information travelling down a parallel line of cables at the same time will not be in a straight line (Imagine a horse race), the further they fall behind the more likely they are to meet the next set of Data(or the next race has already started) or slow that data line down thus corrupting the data, as SCSI speeds reached the 320 point there was less room for skew to occur as the gap between the data was even smaller the faster you pushed the data. this is why serial attatched SCSI (SAS)has been implemented, at min speed of 1.5Gb and future 6Gb speeds and above. seriel data is a stream one following another so skew issues no longer apply.
I hope this makes sense and answers your question