What is the difference between assessable and gradable in CoursePress?


Can someone please explain me a little about the terms ASSESS and GRADE and all their forms that are used in the plugin. I do not understand what is the difference between them. I am new to CoursePress 2 and it is confusing.

For example I do not understand these strings for the grading systems:
total acquired grade % total number of gradable modules
total acquired assessable grade % total number of assessable modules

Thank you.