Tell me about declare pages?

A declarative page is a clipboard page created by execution of a declare pages rule (Rule-Declare-Pages rule type).
Benefits:
Declarative pages can improve performance and reduce memory requirements when all or many requestors in an application need to access static information or slowly changing information.
Notes:
Ordinarily, declarative pages do not persist; they are not saved to the PegaRULES database but are reconstructed when next accessed and the previous version is expired.