What scope would you apply for a Custom Dimension that collects data for users who log in to your website?
When a user is known to your website or application, you should send an ID representing that user with all of your Analytics hits, such as pageviews, events, ecommerce transactions, etc., using the userId field.
Read more here: https://support.google.com/analytics/answer/6205850
User-level scope allows you to conveniently group all of a user’s component sessions and hits by a single value. It is ideal for values that don’t change often for a particular use.
Read more here: https://support.google.com/analytics/answer/2709828#example-user