Fator R is the ratio between the last 12 months of payroll (including the pro-labore) and gross revenue over the same period. For service activities, if Fator R is 28% or more the company is taxed under Annex III of Simples, whose rates start at 6%, much lower than Annex V, which starts at 15.5%. That is why many providers tune the pro-labore to reach 28% and land in the cheaper annex. The calculator picks the annex automatically from the pro-labore you enter.