A pension commencement lump sum refers to a lump sum that is withdrawn tax-free, once you crystallise your pension funds.
A PCLS is the 25% of a pension pot that is available tax-free and is taken as a lump sum. It is paid after you have reached normal minimum pension age, which is currently 55.
It is common to withdraw a 25% tax-free lump sum from your pot, and receive the rest 75% of your pension savings as a flexi-access drawdown or as an annuity. The rest of the pension will be taxed based on your Income Tax rate.