Question on Kiha 283 series train formation

[NGT6 1315]

Hello,

 

having had a look both at the Japanese Wikipedia article (which I understood only very roughly and with the help of the Google translator) and the train formation diagram for Kato's model of this train I wanted to make sure I understood both these sources correctly. Would I be right in assuming that there are both basic 6-car sets and 6+4 car ones, with one of the Kiha 283 driving trailers on the 6-car set being coupled to the flat end of a Kiha 282 series car (with auxiliary cab), and the third Kiha 283 driving trailer being coupled to the flat end of the fourth Kiha 283 driving trailer?

 

I admit I do not quite understand the sense of this odd train formation, though .

[bikkuri bahn]

The additional units allow for more seats during weekend/peak travel periods, and also allows the extra units to be detached enroute, for example at Obihiro on the Sapporo-Kushiro route.