how to understand "bool CCRakNetSlidingWindow::GreaterThan(DatagramSequenceNumberType a, DatagramSequenceNumberType b)" ?

[yandaren]

in the CCRakNetSlidingWindow,

bool CCRakNetSlidingWindow::GreaterThan(DatagramSequenceNumberType a, DatagramSequenceNumberType b)
{
	// a > b?
	const DatagramSequenceNumberType halfSpan =(DatagramSequenceNumberType) (((DatagramSequenceNumberType)(const uint32_t)-1)/(DatagramSequenceNumberType)2);
	return b!=a && b-a>halfSpan;
}
// ----------------------------------------------------------------------------------------------------------------------------
bool CCRakNetSlidingWindow::LessThan(DatagramSequenceNumberType a, DatagramSequenceNumberType b)
{
	// a < b?
	const DatagramSequenceNumberType halfSpan = ((DatagramSequenceNumberType)(const uint32_t)-1)/(DatagramSequenceNumberType)2;
	return b!=a && b-a<halfSpan;
}

how the understand the two comparision function, what the 'half span' mean ?