What is the difference between 'Both Libraries' and 'Shared Library' in Photos app?

I see 3 library view options. The first is "Both Libraries", the next one is "Personal Library" and the last one is "Shared Library". What is the difference between these, and why would I Use "Shared Library" over "Both Libraries"? I mainly use Personal Library and Shared Libraries because they have similar icons (person/people), so I figure they go together, and they are the only choices I see when looking at a photo. Thanks.

[Re-Titled by Moderator]