Particle picking consists in all the interactive operations on the tomograms towards the definition of 3d positions (and possibly) euler angles that roughly correspond to the presence of a particle.
In some cases, the on-screen interaction simply consists in the user directly clicking the centers of the individual particles. But in many other geometries, the user will rather design supporting objects, like filaments, vesicles, pseudo-crystals or freely-shaped membranes. For each different geometry type, Dynamo provides a different model class.
After the stage of particle picking, we normally need to extract particles, an operation that crops the particles from inside the annotated tomograms and puts all the subtomogram files inside a single data folder that can be used to design an Alignment project