Hi Adam,
I gone through the doc Analysis Authorization Using Variable Exits | SCN
Document talks about how to fill default values and how to validate user entered values for same variable.
I_STEP 1 - Written to populate default values based on current user
I_STEP 3 - Written for validating , if user remove default values and enter new values.
coming to your requirement.
Since your already maintaining users and there countries in ZCOUNTRY_USER table
Simple, write code in I_STEP 1 for ZSLDTO_EX_REG variable (for this variable remove ready for input) to populate values from ZCOUNTRY_USER based current user and fill the variable
This will solve your issue.
Thank you,
Nanda