ToPackedArray[expr] uses packed arrays if possible in the internal representation of expr.
Using ToPackedArray will not change results generated by Mathematica, but can enhance speed of execution and reduce memory usage.
ToPackedArray is effectively used automatically by many functions that generate large lists.
ToPackedArray will successfully pack full lists of any depth containing machine-sized integers and machine-sized approximate real and complex numbers.
ToPackedArray[expr, type] will when possible convert entries in expr to be of the specified type.
Possible types are: Integer, Real and Complex.
Only machine-sized numbers can be stored in packed form.
The option Tolerance->tol can be used to specify when small numerical values can be ignored in conversion to more restrictive types, and when they must prevent conversion to packed form.
See also: FromPackedArray, ByteCount.