How do you assign a portion of marks to an answer, e.g. in multiple choice, all of the above is correct, but each of the other choices is partially correct?

There is a grading option called right minus wrong in multi select questions https://documentation.brightspace.com/EN/le/question_library/instructor…. There is also a way of adding custom weights to a Multiple Choice Question where you can set different weights for each answer if some solutions are more correct than others.