AssignAchievement moves a member's progress on one achievement, either setting it to a value or adding to it. Earning is scored rather than boolean: the number of times the achievement has been earned is the accumulated score divided by the points the achievement requires, rounded down, so a one-off badge needs a single point while a "solve 50 problems" award accrues progress. A reference confines the change to its own bucket and the total is summed across buckets, which is how a rule that re-runs counts each distinct item once.