Confused about past service costs and actuarial gains/losses

I’m confused as to which factors exactly affect those two. It seems like it’s unclear in Schweser. For example, life expectancy, salary increase assumptions, years of service, and so on. I’m confused as to which of these types of factors would go under past service costs vs. actuarial gains/losses. Please help, thanks!

The only thing that affects (i.e., gives rise to) past service costs is a change in the formula for retirement benefits.

Changes in the assumptions give rise to actuarial gains and losses only.