There are four fundamental types: two Taylor types and two Polymorphs
1) Taylor and Complex Taylor: Taylor is simply a Taylor series overloading the LBNL version of Berz's "DA-package." Complex Taylor is a type which contains the real and imaginary part of the Taylor series. (Types TAYLOR and COMPLEXTAYLOR)
2) A real polymorph is a type which can change back and forth from Taylor to Real at execution time. (Type REAL_8)
3) A complex polymorph can change from back and forth from Complex Taylor to Complex at execution time. (Type DOUBLE_COMPLEX)
The polymorphs are extremely useful since they allow the programmer decide on the type at execution time rather than at compile time.
KNOBS
Polymorphs (real or complex) can also be "knobs" in FPP. A knob is a special monomial which cannot depend on any parameters of the system. This is useful for on-the-fly parameter dependence on external conditions. Click here for explanations