A = student has mastered the concept at a level where s/he can use the new concept independently, as well as teach the concept to others.
B = student understands the concept at a level where s/he can use the concept in new applications with minimal help either by reviewing past notes or by seeking affirmation support from others.
C = student understands the concept at a level where s/he may not be able to use the concept independently in new situations but at a level where s/he knows how to get the assistance necessary to use the concept in new situations.
All the above levels assure that the students can go forth and use the knowledge at a functional level.
With the passage of time and without reinforcement through use, student mastery levels may slip so that an A student may need some review of notes or reminders from others, a B student may need assistance and review to even remember how the concept works, but even a C level of mastery is a level at which the student remembers the essence of the concept and with minimal review and reflection will move back to a level at which s/he can use the concept in new applications.