what is the difference between black and grey squares which present in front of each Reputation ?

green squares represent the rep given to you by someone that increased your total number of rep points.
black squares represent the rep given to you by someone but that didn´t afect your total rep points number.
there are also red squares which represent the negative rep given to you and that have decreased the total number of rep points.
i will add this to rep system anouncement topic so that there are no more doubts.
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