Compare Blocks on STEP7

Hi, does anyone know what means "Compare Blocks" in STEP7?

I was triying to go online and monitor with a S7-300 when a message was displayed. It said that the project on the PC was different from the one on the CPU and asked if I would like to Compare.

The program in the controller is different or at least time stamped different than the one your trying to go online with. When you compare blocks, there is a great feature, that changes your view to split screen and allows you to see the differences.

Also check to make sure you opened the latest revision of the program. If the last guy downloaded version 5 and your trying to go online with version 4 you won't have the revisions he made to the program to look at online.