Allow you to search all the files that meet list of criteria you enter in the dialog boxes.
LightBox indicate in the document window the number of files that meet the criterias over the total number of files in the catalog.

The dialog contains an 'Action area' which allows you to indicate what LightBox should do with thumbnail that meet the criterias. The actions are :
•Display : LightBox will display only the 'matched' thumbnails
•Select    : LightBox select all 'matched' thumbnails but doesn't change the display
•Mark     : LightBox mark all 'matched' thumbnails but doesn't change the display.

NOTE: 
The meaning of 'All' is always 'all the curently displayed thumbnails'