iMIS REST API interfaces vs IQA?

The API is a bit confusing. It's not entirely clear when to use the generic REST API interfaces vs a custom IQA query?

I am having a lot of trouble finding endpoints that equate to the following pieces of data?

For the following data fields, which do I need?

A user / member / person paid through date (client calls it - "Paid Thru")?
A user / member / person iMIS number (client calls it - "iMIS Id")?

Is it required to use an IQA query to fetch such a pieces of data?

The closest thing I can find is in GroupMember->JoinDate.