Although it is necessary to choose one, this field is not relevant in this use case. We will set it to Summary for demonstration purposes.
Target field
Select a User Picker custom field.
Rules
Add one rule for every possible combination of the fields, assuming that none of the fields can be left blank.
Select Logical expression
Expression
For every rule, evaluate the different possibilities with appropriate logical expressions like the following one.
"Fruit" = %{issue.cfaaaaa} and "Apple" = %{issue.cfbbbbb}
Please, replace the field codes%{issue.cfaaaaa} and %{issue.cfbbbbb} with the ones from the appropriate fields before testing the post function.
Target field value
Enter the username of the user that should be assigned to the ticket when the logical expression is true.
user.one
If you need to add several users to a User Picker (multiple choices) field, select the option Evaluate all rules and add a plus sign "+" before every target field value.
+ user.one
Every user that has an associated matching rule will be added to the field.