A parallel-line assay is the classical method to calculate a relative potency for a dilution assay. It is a linear fit which covers only the (near-) linear portion of the dose-response relationship without their asymptotes.
PLA 3.0 supports parallel line assays along with additional functionality to determine acceptable regions of the dose-response curve (configuration optimization).
The parallel-line method is a robust biostatistical analysis method based on D.J. Finneys Statistical Methods in Biological Assay, 3. edition, 1978. It is supported by the European Pharmacopoeia and the US Pharmacopeia.