If you click on Comparing Diagrams, with a diagram selected, you will be prompted to select another diagram file to compare against. When you have selected one, you will see the current diagram with a number of blocks and/or arrows marked with letter codes in circles.
The letter codes are as follows:
In the case of deleted blocks in the current diagram, block(s) involved will be shown in grey, with the word "ghost" showing below it/them.