I would like to add an opinion only in hopes of furthering a discussion on the subject, and certainly NOT as a proper definition of any of the terms...
In a 'penjing landscape' [there are many forms of penjing other than landscape], stones are often used as not only a means of containing the soil but as 'points of interest' to aid in 'painting an image'...There are also penjing landscape forms that are stones only [no trees] and trees only [no stones]...Mud figures are not a requirement, but are often used to help with the illusion of scale or to convey the essence of a particular place or theme...From the Chinese perspective it is much more important to convey a 'feeling' or relate a 'story', than it is to follow pre-conceived 'guidelines' of what is accepted or not accepted...There is much more freedom of expression in the work, and even the design of the trees shows more freedom...
The 'saikei' form was originally conceived to use material that was in development and in need of more growth to create a 'landscape' which could be proudly displayed in ones garden, as the trees were being developed...The stones of a well designed saikei will be equally important, if not more so, than the trees...Mud figures are seldom used, and even considered undesirable...Saikei deals more in character with representing 'nature without the evidence of man'...
'Hon Non Bo' from the Vietnamese culture usually features the stone or 'mountain'...The trees serve the purpose of accent or a device to emphasize scale...These plantings are usually displayed in a shallow tray filled with water to represent the familiar 'mountains in the sea' rock formations found in the area...Mud figures are sometimes used, again to help with the illusion of scale...
I hope others will also share their opinions and knowledge on this subject so we all may learn...I was not born into any of these cultures, nor have I been educated in them, however I do have an interest in learning the intricate details of each form, in order that my own designs can be better...By better understanding all forms of 'artistically potted trees' we should become more versatile in designing our own creations...
Regards
Behr