Marginal relief
From ACT Wiki
Tax - UK Corporation Tax - UK Capital Gains Tax.
Marginal relief is tax relief designed to bridge a boundary between (1) a full rate of tax - or charge to tax - and (2) lower rates, or tax free transactions.
Applying marginal relief has the effect of a lower overall effective rate of tax, or a lower tax charge.
Examples of contexts where marginal tax relief have applied include UK Corporation Tax, and UK Capital Gains Tax.