Make not fish of one, and flesh of another.

Do not sort comparable people into unequal classes or grant them different treatment without a fair reason. The saying warns against drawing a divisive distinction between those who ought to be judged by the same standard.

It can also condemn partiality in decisions affecting several people. Its central point is not food or trade, but the unfairness of treating one person as exceptional while imposing another rule on someone else.

Use it when criticizing inconsistent or preferential treatment among people in similar circumstances, especially where equal rules or impartial judgment are expected. The figurative wording may be opaque, so a brief explanation can help readers unfamiliar with the expression.