interesting article, page-9

The 5% of revenue royalty is to cover projects if, for some reason, they are not very profitable (eg falling oil prices). It guarantees the govt an income every year.

However, we know the Tui project to be extremely profitable, therefore the 20% of net accounting profits will be the figure levied as this number will be considerably bigger than 5% of revenue.

It's not an optional choice. The govt will levy whatever is the larger result. So for Tui, 20% of net accounting profits it will be.