IAS 19 employee benefits

dear Sir,

I watched the SBR lectures re employee benefits and have the following questions;

1. interest on defined benefit obligation
in simple scenario interest is calculated as % * opening balance. however, there are instances where interest is calculated as & * opening balance – benefits paid. could you kindly clarify when to use what?

2. interest on plan assets

in simple scenario interest is charged as % * opening balance of plan assets. however, there are instances when this is calculated as % * opening balance of plan assets + contribution – benefits paid. could you kindly clarify when to use what?

thanks a lot