Question: what is historical part of tree?

RE this statement:
"Geni Pro subscribers now have full permission to add on to, edit, and merge profiles in the historical parts of the tree. Non-Pros can no longer add new profiles to the historical tree or merge profiles."

What is the definition of the "historical tree"?
Based on birth/death dates?
Based on generations from me?

Do my privacy (i.e. public/private) settings still apply in whatever is the historical part of the tree?

I looked for an answer to this question but couldn't locate it myself.

Thanks for helping me clarify! I am a Pro user but I help support many of my non-Pro family members.

Thanks in advance.