Drop Stops are designed for situations when one Stop (Stop A), must happen before another (Stop B), but the rest of your Route order can vary. This allows the Route to be optimized within those constraints.
To set up a Drop Stop situation where Stop A must happen before Stop B:
Select the three vertical dot icon next to your Stop A in List View
Select Add Drop Stop.
Select your Stop B from the popup list.
Then, when you Optimize, Stop A should come before Stop B in your list.