Reducing balance applies an annual rate to amount of Depreciation to be applied, ie if you had a capital sum of 100k, and depreciated using the reducing balance method at a rate of 25% per annum, year one you would depreciate by 25k, year 2 18.75k (25% of 100k-25k), year 3 14.06k (25% of 100k-25k-18.75k) and so on.
For capital allowance purposes, it’s the same principal, except the reducing balance is used as tax relief