Microsoft XML Core Services (MSXML) 5.0 for Microsoft Office - XSLT Reference

NaN Values

The term NaN, which stands for "not a number," means a symbolic entity that represents a value not otherwise available in floating-point format.

There are two kinds of NaNs.

Quiet
Represent unknown or uninitialized values.
Signaling
Represent symbolic values and values that are too big or too precise for the format. Signaling NaNs raise an invalid operation exception whenever an operation is attempted on them.