I asked a lot of questions about nested tikzpicture environments and if I agree with Andrew and Martin that nesting TikZ pictures isn't a good idea, it's interesting to know why. I gave some examples where it looks possible to nest TikZ Pictures.
With this examples I discovered or created some useful macros like
\ensuretikzby Martin here
\restorefontcreated by Martin here
\restorecurrentboundingboxcreated by me here
7) it's interesting because the definition is
These macros are useful to manage the bounding box and the font but perhaps other values are interesting to save and restore.
It's possible to look at the sources but it's not very easy for a lot of us. The aim of my question is to build reference answers to describe what are the variables set when tikz picture begins and what are the differences with a scope?