Why there are different plans
UK student loan repayments depend on which plan applies to the borrower. Different plans have different earnings thresholds, so two people earning the same salary can still have different loan deductions.
Why postgraduate loan is separate
A postgraduate loan is treated separately from the main student loan plans. In practice, someone can have both, which is why Tax Helper includes a separate postgraduate loan switch instead of burying it inside the main dropdown.
Why deductions start only above a threshold
Like tax bands, loan deductions only apply above the plan threshold. This is why users sometimes see no change at all until their earnings move into the repayment range.
Why loan deductions matter for job comparisons
Student loan deductions can materially change take-home pay. A pay rise may feel smaller than expected once tax, NI, and loan repayments are all taken into account. That is exactly the kind of question a calculator should make easier.