Different between 'site keyword' and 'page keyword'

First of all, this module was really awesome. In this module, you used two different term and of course it would return a different function and target which are;

• site keyword
• page keyword

so, in term of SEO impact and its function, what make it different for these both? what should i tag in for one articles? when should i used site keyword compare page keyword?

from my though, page keyword is subset for site keyword. does it really need to have separate function rather than one function that will apply to the whole site. thanks in advanced.