Automated online formative assessment of students' work in a rich digital environment has the potential to support and develop students' reasoning process. Previous studies have presented the challenge of designing e-tasks. Here we focus on the challenge of designing a personal online formative assessment that supports the students' reasoning process. A common type of online formative assessment is elaborated feedback. We provide a design principle for elaborated online feedback of students’ work on an online example-eliciting task (EET) using the Seeing the Entire Picture (STEP) platform. We demonstrate two cases of attribute isolation elaborated feedback (AIEF) design, and the case of a pair of students who used the AIEF to support their reasoning process.