Minimum assay: we can understand what the minimum assay % stands for just from the context. For example, if you found a label of a bottle as Carbonate sodium 20%, it is mean that the solution is prepared by dissolving 20 g of carbonate sodium into a total volume of 100 ml (including the volume of the added amount of carbonate sodium)
Maximum assay: The quantitation limit of an individual analytical procedure is the lowest amount of analyte in a sample which can be quantitatively determined with suitable precision and accuracy.