Parallel and series cut crystal difference
Every crystal that I know of resonates at two frequencies, one due to internal series components and one due to parallel series components. Additionally the two frequencies are very close.

So if one has markings on it that indicate it is series cut then running it in a circuit that naturally only works at a parallel resonance means that the xtal accuracy is called into question; not much but enough in a critical design to affect performance.

In other words, the PIC will force any xtal to run at parallel resonance (because it just won't oscillate at series resonance).