Hi Sandeep,

The Pega Help says "In the context of an embedded page, identifies the top-level page to which the current embedded page belongs." explanation for the top keyword over here. Hope it might help you.

Regards,

Mahesh M

Please check below link. Search for TOP keyword for related information. 

https://community1.pega.com/sites/pdn.pega.com/files/help_v718/procomhelpmain.htm#concepts/concepts2/pagenames.htm#kanchor5202

Below two links may also provide some help:

https://community1.pega.com/community/pega-support/question/using-top-keyword-data-transform

https://community.pega.com/knowledgebase/articles/declare-expression-rules-using-top-or-parent-keyword-depend-existing-pages