The datatype whose representation is known are called manifest types or concrete types.
Manifest types refer to types whose representations and behaviors are explicitly known at compile-time or runtime, allowing the system to understand how to store and manipulate the data.
Concrete types generally refer to specific instances of abstract data types or classes where all the details are fully defined, as opposed to abstract types which may have incomplete definitions.
In some contexts, static types can also be used to describe types that are known at compile time, as opposed to dynamic types which are determined at runtime. However, "manifest types" and "concrete types" are more precise terms for the concept you're referring to.
