Setting up a Custom Scale for Assessment : the SOLO Taxonomy scale
You can set up your own custom Scale (under Course view (on the left side) -> Grades -> find the tab labeled Scales -> Add a New Scale button at bottom)
Put " SOLO Taxonomy " for the name of the scale, and use Pre-structural, Uni-structural, Multi-structural, Relational, Extended Abstract for the Scale Values (separated by commas). In the Description you could briefly describe the SOLO Taxonomy, and give a link to a reference.
Then, when you create a Forum activity, you can select Aggregate Type -> Maximum Rating (or Average Rating), and Grade -> Scale: SOLO Taxonomy.
Then as a Teacher, you can rate posts using the SOLO Taxonomy as rating choices, which the student will see.