Why to create a NetClass?
Net Classes are very useful during layout – by one click you can select whole interface (e.g. UART1, SD card, …. ) or a group of special signals (DATA BANK 0, 1, 2, …). You can also apply some rules on particular class (e.g. minimum track width, clearance, length, …)
Why to create a Differential Pair Class?
Differential Pair Classes are needed in case you are setting up rules for differential pairs. Differential pair rules will not work with standard NetClass.
This video explains how you can set up NetClasses and Differential Pair Classes