this problem is haunted me long time: when subdividing just a selected block then close block is affected too with nonsense slices, BUT how to subdivide without slices?
on picture there is red marked slices that i do not want when subdividing but selected area is ok like i want. This is very nasty thing sometimes. I do not know Blender fully so maybe i have missed option to subdivide without side effects.
Currently blender only permits faces to be made of 3 or 4 sides. Those extra edges are needed to permit this otherwise you’d have a five sided face created. You can look at using a bmesh build from graphicall.org but still pretty buggy