In general terms, the symbols in the KiCad library can be categorized into two groups:
Generic symbols can be used with multiple footprints, and do not have a default footprint assigned. Generic symbols allow flexibility in the design workflow. Symbols are first added to the schematic, and the footprint is selected before PCB layout. Using generic symbols allows a small number of library symbol elements to represent a very large number of possible component combinations.
An example of the use of generic symbols is the Resistor and Capacitor symbols available in the KiCad library. These symbols do not have an assigned default footprint, as there are many possible compatible footprints.
Atomic parts fully define a component, specifying a matching footprint, and are named based on the
MPN (manufacturer part number). Atomic parts are ready to be placed onto the PCB as they are already associated with a footprint.