PIN code VS account password

PIN code is only the password for you computer. Unlike what the name might suggest, it can (and should) be composed of numbers, letters, symbols ...

Each computer you log in will ask another PIN code.

Your account password on the other hand, is the password to your Microsoft account, on the web.

Where do I use which?

You would use your PIN code to log in to your local session, on your computer. And that's all.

You use your account password to connect to Office, Outlook, Sharepoint ...