This is documentation for Mathematica 6, which was
based on an earlier version of the Wolfram Language.
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MathLink Packets
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
ReturnPacket the final result of an EvaluatePacket evaluation
    
EnterTextPacket a string to be entered like keyboard input
EnterExpressionPacket an expression to be entered like keyboard input
ReturnTextPacket, ReturnExpressionPacket final results returned
    
TextPacket Print and other incidental output
MessagePacket name of a Mathematica message
    
CallPacket data for calling functions in an installable MathLink program
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