Hi Drew,
thanks for you reply. Yes I tried and it works, executing the function as a different user using “Change User”.
What confuses me is the following.
According to the docs and Stitch UI, I shall be able to choose how to authenticate a function. For example, this is what the docs state for User Id - “This type of authentication configures a function to always run as a specific application user”.
Why can I set this if it is not used? Maybe more info in the docs will be good.
To clarify why I need this to work:
My use case is that I would like periodically to send emails to my users. For that I need their email addresses. The email address of a user is stored in the internal stitch database to which I don’t have direct access (except with Stitch API). I was hoping that I can use the “Script” authentication method when I run my “sendEmail” function. The script should get the parameters of the “sendEmail” function and return the user id to which I want to send the email to. Does this make any sense ? 
Thanks again!