When there is no current page selection, the "Selected Pages" option will be unavailable.
If the current output type is set to "Create New Searchable PDF" a new document containing only the pages from the selection will be created. OCR Pages…recognizes the text of the previously scanned PDF document to make the scanned text searchable and selectable.
Selecting this menu option opens the OCR Pages dialog:
In addition to selecting pages for OCR, this dialog allows you to temporarily change the OCR settings for the current task, if you wish, without saving them for next time. The options available under Recognition and Output are the same as those found in the preferences. Please see the documentation for Edit > Preferences > OCR for details. You must use the preferences page if you want to save your settings for the future.