This page begins to explain the difference but it doesn't answer the questions posed in the issue title above.
What is the difference between the two
How can pip install -e . install my package if it's not on pypi?
This may be related to the environments discussed in another issue.
cc @chenghlee
We might want to add this content on this page
And also it might be good to consider personas which would help a user find the things that they need to know ...