How many pounds is the dropdown incremental weight on the Optima Series machines at Planet Fitness? I have no idea how much weight it adds because there is no number on it.
I'm assuming that you're referring to the weight stack of a selectorized weight machine:
When these are labeled like the image above (1, 2, 3...) the only way to know the weight is if the gym has listed it on the machine. We can look at the technical specs of the Optima series equipment (or any equipment) and find that the stack weighs 202 lb but that weight isn't divided equally. Commercial gyms can also add more plates to serve more clients.
The first weight plate is usually around 25 lb and successive plates add 10 - 15 lb. This isn't a rule of thumb.
The reason a lot of these plates aren't labeled with weight is that the weight isn't actually accurate. For example, if you installed the above weight stack on a single-pulley machine it would be 1:1 for weight, but if you installed it on a double pulley, everything would read half. It isn't useful to label #10 as 100 lb because it might be less (50 lb).
There is a drop-down incremental weight on the Optima series, which is now what I think you're referring to. There's only one drop-down weight and the weight stacks are still numbered so it should logically follow that it is a half-step.
So, if you're working out with #8 and #9 is a bit too hard, you can pull down that weight to make it #8.5 and progress with smaller jumps. If the plates are 10 lb, that would make that increment 5 lb. If the plates were 15 lb, then the increment would be 7.5 lb. If there were two increments, the plates are likely 15 lb and the increments are 5 lb (i.e. 15, 15+5, 15+5+5, 30).