These are restrictions on the types of array elements. Typically if an array's elements are to be communicated, they must have POD type. The idea of a POD type is defined in the ANSI C++ standard. Informally it is any type that can be copied to a byte array by standard operations such as memcpy.
Schedules that perform arithmetic operations or comparisions will impose further restrictions on the types of the array elements. In the current library all such schedules are template classes, parametrized by the element type. The template argument can only be instantiated to a type that supports a suitable set of arithmetic operations.