Label
From OpenFOAMWiki
Variable type label is defined in OpenFOAM for storing integer data. It is widely used throughout OpenFOAM's source code wherever one might commonly expect an int.
Since the C++ standard does not define the sizes of variable types in bytes, the maximum value an int and other built-in integer types can take is not strictly defined by the language. OpenFOAM defines the maximum size of label to be at least 2e+9 or 9e+18 depending on whether a 32 or a 64 bit machine is used. This is implemented by looking at the maximum values of int, long and long long on the current machine and using typedef to define label as a type capable to fit the desired range of values.
