Symbol references are used to identify parts on schematic diagram. The PIC micro below has a symbol reference of U1. It is linked to the associated footprint on the PCB.

 

Each part has it's own unique symbol reference. This consists of a 1 or more characters ( e.g. the U in the PIC micro shown above )  and a numeric postfix.

 

A part can consist of 1 or more sibling schematic symbols e.g. a 7413 TTL as shown below.

When a part has 2 or more sibling schematic symbols, each symbol reference has a further numeric postfix. For instance, in the 7413 the prefixes are '-1' and '-2'. These are automatically generated for you.

 

You can renumber the symbol references using the symbol reference renumbering  tool.

 

You can edit the properties of a symbol reference using the symbol reference properties dialog in the properties panel.