Difference between Virtual Users start sequentially and Ramp Up Load Variation Policy?

To ramp up virtual users over a period of time, I see two options.

First, in Advanced Population Parameters, there's an option for virtual users to start sequentially, over a period of X seconds.

Second, in Load Variation Policy, there's the Ramp Up option, where the initial user number, increment, and maximum users can be chosen.

What is the difference between these?

Thank you.