All the forces mentioned in the slide are "subjective" forces, and there might also be "objective" forces, like wind, gravity, and upthrust force due to wing flapping and different air flow speeds.
Reading through the source I can't seem to find an answer to this question. How would we know which bird, or particle, takes a certain behavior? Off the top of my head, I was thinking that we can probably distribute these roles randomly. We can then limit the amount of roles. For example, we could have more birds in alignment behavior than we have in repulsion or attraction. I'm sure there is probably some kind of more complicated implementation, since I think some issues with this could be too many birds of the same type being grouped together
@DanielMolina24 I actually think each bird with take all of these roles at once. So they would both be experiencing attraction to the center, repulsion to the neighbors, and alignment towards the average trajectory. What determines the behavior is how large one force is compared to the others based on the current position of the bird.