Once again, thanks all, geek745, mrcode, orangesky, chrism01, MTK, jmite, mostlyharmless.
(I'm late responding cos have been busy clean-installing ubuntu from disk as my 'workaround'to a persistent hang in Mandy, such is life) [in which BTW I discovered vital disruptor to disk's proper partitioning was not knowing to unmount hard drive before diskinstall]
Altogether, these replies have greatly cleared my confusion way back from 100%, which is great! Some of it still is not clear, but there's plenty in there to work with, hierarchy data and much else besides, and it's all on file for continued repeat reads. Very helpful, thanks for your time and work, great.