What each one does
Kappa forms strong, brittle, slightly brittle gels and needs potassium. Iota forms soft elastic gels, needs calcium, and rebuilds itself after shear, which is why it survives pumping. Lambda does not gel under any conditions and simply thickens.
What decides it
Whether the product is set and left alone, set and then disturbed, or never set at all. A dessert in a pot takes kappa. A sauce that is pumped after setting takes iota. A drink takes lambda.
Where the obvious answer is wrong
The obvious answer when a kappa gel is too brittle is less kappa. That gives a weak brittle gel rather than a strong elastic one. The right move is usually locust bean gum beside the kappa, or a shift to iota.
