if a pupil is to attend a PRU full time for a limited period of perhaps a few weeks then return to the mainstream school the PRU would be the subsidiary establishment
os bydd disgybl yn mynychu UCD ar sail amser llawn am gyfnod cyfyngedig o ychydig wythnosau efallai, ac yna'n dychwelyd i'r ysgol brif ffrwd, yr UCD fydd yr 'is-ysgol'