It does depend on the category and your financial situation though - for example, if you have been working for the same employer for 6 months or more, but your income is variable, you may have the choice of A or B.
For example,
For Category A, if the income is variable, they will take the lowest payslip for the 6 months and use that to calculate the annual income, so if one payslip is below £1,550, you won't qualify for the visa and will have to go with Category B.
However, if all the payslips are above £1,550 but do vary, even taking the lowest one will still give an annual income of more than £18,600 so you can either still qualify under Category A but only have the lowest payslip used for the calculation, or you can apply under Category B and show 12 months of evidence if you want to.