ld vs sd on cac/ciss/aac and friends

[Chris Ross <cross+netbsd%distal.com@localhost>]

  I am building a machine with a ciss(4) in it, to replace an older  
Compaq that has a cac(4) in it.  I noticed, and was surprised, that  
the ciss(4) presents the volumes as sd(4) devices rather than ld(4)  
devices.  My cac's and aac's present ld(4) devices, which makes sense,  
as that's largely what ld(4) means.

  So my question first is, why is ciss(4) different.  And more  
importantly, assuming "just because it is that way" might be the  
answer, is there any notable advantage or disadvantage, in provable  
reality or in theory, of doing it one way vs the other?

  Thanks.  Just an "inquiring minds want to know" kind of question...

                       - Chris