What are planar boards? you know the real information about it?

In the personal system /2 line the motherboard is much more like a true single board micro computer which has an added expansion bus,one so radically different from its predecessors that IBM describes its expansion bus as a channel .to the function contained on pc-style system boards,the ps/2 adds video circuitry input and output ports,and floppy disk control
circuity.
IBM highlights the updates and upgrades that seprate the ps/2 motherboards from those of previous computers by referring to the motherboards of ps/2s as planar boards,often dropping the surname the new term is both descriptive the flat board is topologically a plane and in common use to describe similar assemblies in other electronic applications(as well as other computers) the term was sometimes used by IBM to describe the system boards of previous computers and the term "system  board"is occasionally used to describe ps/2 mother boards,planar appears to be the consistently preferred choice for ps/2 motherboards, however.
Although labeled by just two descriptive terms,IBM mother boards are quite diverse every ps/2 model including those with  similar electronic abilities like the models 50 and 60 has a distinctively different planar.even those machines with the  same model number may have different system boards,for instance each of IBM's three models 70 has its own planar board  design  PCs,too use variant systems boards for each model except one,only the xt and portable personal computer share an identical system board.PCs on the other hand have gone through two major design changes..