SADB and SPDB implementation

I have a question about the description of Micheal Richardson about KLIPS
Structure at document
http://www.sandelman.ottawa.on.ca/SSW/freeswan/klips2req/klips2req.html
If every trasform has an own SPI and so an own SAID, I don't understand the
need of linking the different tdb of a same spigroup by field inext and
onext.Also the field hnext seems to me useless, if every tdb is indipendent
accessible by his own SAID.

Someone can help me? Thanks