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Actuarial Gains and Losses and Plan Amendments are always included in PBO, in their total amount.

However, they are not recognized in the I/S immediately. Instead, they go directly into OCI.

Deferred Gains/Losses in OCI = Actuarial Gains/Losses + Difference between Expected Return and Actual Return on Assets

When the accumulated value of Deferred Gains/Losses in OCI exceeds the 'corridor' (=10%*max(PBO, PA)), you must amortize the excess over the corridor, over the remaining life of the plan. This amortization amount shows up as your "amortization of deferred gains/losses" in pension expense.

Plan Amendments also go into OCI, and this shows up as "amortization of prior service costs" in your pension expense calculation.

Note: under IFRS, any prior service costs that have been vested go directly into the I/S.

So remember: PBO is your OBLIGATION - it has to include the total impact of any plan amendments or changes in actuarial assumptions. But Pension expense is your PERIODIC expense - so that is why you only include amortization of these amounts (in an attempt to "smooth" pension expense and reduce volatility).

Also, under GAAP, the net pension A/L on the B/S is simply the funded status (PA - PBO). Under IFRS, the net pension A/L on the B/S is the funded status +/- adjustments for any UNrecognized costs in OCI.

Hope that helps!