All grades indicate a student's level of achievement. In the past, it was customary to include participation and level of completion in a student's grade. Now, the common practice is to give a grade based solely on a student's mastery of certain material. Therefore, grades indicate proficiency and a plus or minus suffix indicates (to a more precise degree) the level of proficiency.
Plus and minus suffixes allow for more precision in grading because they create tiered levels within each level of overall proficiency. For example, students receiving an A, 4 or check mark know when they receive a minus symbol beside that mark that they are not yet at the top of the proficiency scale. They know they can still do more to improve. Conversely, students receiving a plus symbol beside the grade mark know that they have mastered that proficiency level.
Many modern report cards indicate each level of proficiency so that students and parents know what needs to be achieved to move to the next level. For example, students receiving a "3-," "B-," or "check-" know that they barely met proficiency. Typically, the top of the scale is advanced proficiency. The middle represents proficiency and partial proficiency. The bottom of the scale is not proficient. However, a plus or minus sign beside the major grade indicator reveals proficiency level to a finer degree.
Many teachers place a plus slash minus to further indicate a student's achievement level. For example, a "B" plus slash minus indicates still further achievement towards an "A" grade. The plus slash minus grade is in between a plus and a minus. Using plus slash minuses as part of a grading system effectively gives twelve possible grades students can receive and allows for more personalized feedback than simply using letter grades, check marks or numbers alone.