Extendable limits implement the IExtendableLimit interface which defines the ExtendableValue property. The meaning of the ExtendableValue is different for each implementation. For instance the Use limit assigns the Uses property as an alias. The Time limit uses it as a number of seconds.
When an extendable limit expires then, validation will either fail or the user will be given an opportunity to extend the license. A license server can automatically extend subscriptions or use an extension code to extend the license manually.
The forms may be customized or completely replaced to fit your specific application style.
See the How to Create a Custom Form topic for details on replacing or customizing the form.