[advisedwang]

There can be quite a few reasons to pull in more domains than necessary:

- Branding. Perhaps Microsoft wants a strong stamp that Sharepoint is the productivity tool and Live is the identity system.

- Using security boundaries. Web browsers respect domain differences as a primary security boundary. If live.com is security critical, they may want to make sure extras are hosted elsewhere so it can't access live.com cookies, make XHR requests to live.com etc.

- Redirects may need to jump through extra domains to make sure that logging in once works accross every property. For example, when you log in to Google Drive it redirects you through YouTube in order to set a cookie on YouTube to log you in there.

- Partners. A company may use a partner for some part of their tech stack, for example MyBiz LLC might use Trust Inc to provide DOS protection. In some cases this might be implemented by throwing up trust-mybiz.com somewhere in the mix.

[asd4]

The security boundaries rationale is interesting. I think I need to learn more about that.