Duties -- Domain Expert's Guide
The duties of the domain expert in the context or making a system specification with RIDDL are to, simply, provide their expertise. Although helpful, it is not necessary to understand all the details of RIDDL in order to collaborate with authors and implementors on the system model’s definition.
Whatever it is that you know as an expert, that domain knowledge is essential to the correct construction of a system model. Make sure that you do know it backwards, upside down, and inside out. Your knowledge from direct experience with the domain will be most helpful. For example, if you only know rocket science from the perspective of theoretcial physics text books, your expertise will be theoretically correct but practically useless without hands on experience of building rockets that successfully reach their objectives.