When exchanging expressions with external programs, the
Mathematica kernel uses the convention of wrapping the expressions inside packets which identify what role the expressions have, or how they should be processed.
EvaluatePacket — an expression sent purely for evaluation
EnterTextPacket — a string to be entered like keyboard input
EnterExpressionPacket — an expression to be entered like keyboard input
TextPacket — Print and other incidental output
CallPacket — data for calling functions in an installable
MathLink program
