题目是：
After income tax, Mr X has ￡1000 of the 20% tax band left unused. He makes a single chargeable gain in the tax year of ￡15,000. how much tax does he pay?
我知道怎么算，可是我就不能理解了，tax band 不是税阶吗？？那个“1000 of the 20% tax band left used” 是怎么产生的？？怎么会有税阶没被使用？？这种情况是咋发生的？