Service/center Lifetime user you need to login to Service Center, Life Time, Service Studio and Integration studio.
It is the account the developer uses on a daily based to develop applications.
<domain>/users are the user accounts that the people need to be able to login and use and OutSystems mobile or web application.
Apart from that you also have your OutSystems community account (to login on OutSystems.com for learn, forum, Forge, etc.)