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A subscript or superscript is a number, figure, symbol, or indicator that is smaller than the normal line of type and is set slightly below or above it.
Answer Wiki. The answer is: Whatever the author wants! There are some conventions that most mathematicians follow - such as a subscript denoting a Tensor in a Physics derivation, or in Numerical Solutions a subscript usually denotes the spatial-step and the superscript the time-step.