Mathematica allows arbitrary hierachical namespaces. However, several standard "contexts" are conventionally used for predefined purposes.

"Above" is a front end token that creates an OverscriptBox and fills the base with a selection.

"Balance" is a front end token that expands the selection to cover the nearest pair of matched bracketing characters.

"Below" is a front end token that creates an UnderscriptBox and fills the base with a selection.

"DebuggerStep" is a front end token that stops the evaluation at the beginning of the next expression.

"DebuggerStepOut" is a front end token that stops the evaluation at the end of the current stack frame.

"DuplicatePreviousInput" is a front end token that duplicates the contents of the nearest input cell above.

"DuplicatePreviousOutput" is a front end token that duplicates the contents of the nearest output cell above.

"Evaluate" is a front end token that evaluates the selection in place.

"EvaluatorAbort" is a front end token that aborts the kernel operation in the specified kernel.