Hi Barrie. Poncirus is a great rootstock for dwarf tree's & cold hardiness but is not very vigorous. Carrizo and Swingle are a more vigorous rootstock for warm climates & certain soil types.

Carrizo as a rootstock produces large trees with high yields and produces large fruit with fairly good juice quality. Used many for grapefruit, mandarins and sweet oranges. Does poorly on clay soil and soils high in calcium. - Millet

Carrizo also can handle a higher soil pH than pure Poncirus (although still likes it somewhat acid). Before Swingle became popular, it was the most popular rootstock under Florida citrus for a while. Then when it became realized that it is nearly as susceptible to blight as is rough lemon, it lost some of the popularity, and the popularity of Swingle increased. We still use some Carrizo, though.

It's easy to mix up with 'Troyer' citrange -- they are utterly identical looking, and some California literature suggests that they are the same cultivar, with two names. However, in Florida, 'Carrizo' has shown considerable resistance to burrowing nematode, while 'Troyer' is completely susceptible. In all other respects, though, they are identical.

Barrie - you ask 'what's the point of this hybrid?'.
Well, the original idea of crossing Citrus and Poncirus was to try and develop an eatable, frost hardy fruit. Eatable (not just edible) means reasonably sweet.
It never really worked, although some citranges are better than others. Perhaps, as more crosses are made the dream will come true.
However, as others have said, along the way it was found that citranges had advantages over Poncirus as a rootstock in particular circumstances.

Lot's of great info again folks. Now the picture is getting clearer.

Cheers, Barrie.