Could you please explain it more elaborately?
The page parameters are evaluated upon entering a page. The value is assigned as mapped by the value expression. In your example, the page parameter
customerAccountId will set the
customerAccountId in the
customerAccountHome component. This assignment happens prior to the rendering of the page so that the
customerAccountId can be accessed from
customerAccountHome. One interesting thing is that this operation is not uni-directional. The value of
customerAccountHome.customerAccountId is read by the page parameter. So it is both setting and getting the value of the value expression in that order.
Are they invoked on any method call or on specific method?
The page parameter expressions are evaluated for every s:link or s:button you use in your page. It sets the corresponding backing bean property on every method call to a seam component (unless you specifically avoid it by the annotation @ByPassInterceptors).