EXTRACT BODY is used to replicate a body that is defined within the same part-file. Use TimeStamp to allow the positioning of the EXTRACT BODY feature within the History Tree of the Part Navigator, otherwise, the EXTRACTED BODY feature will be pushed down the History Tree whenever features are added to the original BODY.
WAVE GEOMETRY LINKED BODIES are created in the current Work Part and are (usually) linked to a body in a different component within the assembly.