Thanks melanie for the explanation. It help me verry much. If have some projects in my mind where I could use this knowhow. For example:
If have a master PIC wich sends datastream to several slave PIC's (8 or so) they filter and use the data to do something with it. When they share/use the same oscillator stream I could get the slave PIC's to do there task at exactly the same time (when al other slave PIC are ready to)

Again thanks.