The Defined Benefit Obligation (“DBO”) or Liability is the discounted present value of all expected future obligations resulting from service rendered by the employee, up to and as at the valuation date. The DBO excludes expected future service.
Calculating the DBO requires the use of an actuarial valuation method, typically the PUC. Once the Lifetime Liability has been calculated, the Net Periodic Pension Cost is obtained by dividing the Lifetime Liability by Expected Total Service. The DBO is then calculated by multiplying the NPPC by past service (or deducting from the Lifetime Liability the NPPS times expected future service).
Find an example here.
The NPPC is not necessarily equal to the CSC.
If there are any corresponding scheme assets, they are taken into consideration by deducting the fair value of those scheme assets from the present value of the DBO.