As I am new to Nintex workflows, this is a very basic question. What does the red text in the attached/uploaded screenshot refer to? Does it represent an error/issue, workflow variable, or something else?
it would imply that the value was selected from the lookup button (the address book with the arrow) which can access common variables, item properties, Nintex constants, workflow variables etc. If you click the address book you should be able to see which one was selected. Alternatively if you switch to CAML query in that action you will see which type it is (i.e. item property or variable).