Particle system own force field system vs external force fields

HI, I’ve been learning about particle systems and am puzzled by there being options for a particle system’s own force fields (limited to two force fields), and external ones (unlimited). I haven’t been able to find any discussion or explanation of this online. Is there any difference or reason to use one or the other? The only thing I can think of for the emitter’s own force field is that it can move with the emitter, but you could achieve the same thing parenting an external one to the emitter.

Thanks, Pete.