Tax relief arrives in two structurally different forms, and the difference determines who benefits and by how much. Treating them as interchangeable produces consistently wrong conclusions.
They act at different stages
A deduction is subtracted from income before tax is calculated, so it removes an amount from the taxable base rather than from the bill.
A credit is subtracted after the tax has been calculated, reducing the amount owed directly, which makes its value independent of the taxpayer's rate.
The sequence is what produces every other difference between them, including who gains most and whether the relief can exceed the tax due.
Deductions are worth more at higher rates
Because a deduction removes income from the top of the stack, its value equals the amount multiplied by the taxpayer's marginal rate.
The same deduction is therefore worth substantially more to someone in a high band than to someone in a low one, and worth nothing at all to someone below the threshold for paying tax.
This is why reliefs delivered as deductions attract criticism as regressive, and why some systems restrict certain deductions to a fixed rate regardless of the taxpayer's band.
Credits deliver a flat amount
A credit reduces tax by its face value for everyone who can use it, so it delivers identical benefit across income levels.
Governments use credits where the policy aim is a uniform incentive, such as encouraging a specific behaviour that should be equally attractive to all taxpayers.
The design also makes the cost easier to forecast, since the revenue given up does not depend on the income distribution of those who claim it.
Refundability decides who is reached
A non-refundable credit can only reduce tax to zero, so a taxpayer with little liability cannot use its full value.
A refundable credit pays out the excess, which turns the tax system into a delivery mechanism for support to people with low or no tax liability.
The distinction is decisive for policies aimed at lower earners, since a non-refundable version simply does not reach the group it was intended for.
Allowances and exemptions sit alongside both
An allowance excludes a band of income from tax entirely, which behaves like a deduction available to everyone rather than one tied to specific expenditure.
Exemptions remove particular categories of income from the calculation, and the benefit again scales with the taxpayer's rate rather than being flat.
Because the mix of deductions, credits, allowances and their thresholds differs sharply between jurisdictions and is revised frequently, the current rules should be checked before assuming how a relief behaves.