organic chemistry - How is charge density a useful measure of polarising power and polarisability?

How do you use charge density ($Z/r$) as a measurement of polarising power and polarisability, and hence hard/soft acid character?

So far I've been looking at different polarising power and polarisability and how that affects a substance in whether it is a hard or soft acid/base, but I don't understand how charge density comes into play, and how we can use it to understand the characteristics.