In assay, ``System performance'' should be defined by the resolution between the test ingredient and a target substance to be separated (a closely eluting compound is preferable),and when appropriate, by their order of elution. In purity tests, both the resolution and the order of elution between the test ingredient and a target substance to be separated (a closely eluting compound is preferable) should be prescribed.In addition, if necessary, the symmetry factor of the test ingredient should be prescribed together with them. However, if there is no suitable target substance to be separated, it is acceptable to define ``System performance'' using the number of theoretical plates and the symmetry factor of the test ingredient.