Can you explain Elements to me?

Typically, a VFX Supervisor will break down a shot into all the components or layers needed to composite the final shot. These can be things like a background still, a cleaned up version of the plate, a roto matte, CG renders, particle system renders, etc. Elements can represent this "breakdown" of a shot. Elements are subordinate to Shots, and are intended to represent the various components of a shot. Think of Elements as the layers in a composite, like a background plate, 3D render, matte, etc.

By creating Elements, you can separately track the production of all the things needed to proceed with the composite. Now, you can also do this with Tasks on a Shot, so when do you want to have the added complexity of Elements?

If the production is relatively small, it might be sufficient to track the production of these elements as tasks on the shot.

However, if you have a larger production team, it might make sense to create Elements under the Shot. Say you have a VFX supervisor or producer who will be delegating the production of individual compositing inputs -- for example, you have a roto team with its own supervisor. If you create Elements, he and his team can work down at the Element level, with their own tasks and discussions, while the compositors, VFX supervisor and producer work at the Shot level. When all the elements for a Shot have been finalized, then compositing can be proceed.

Elements have another use: Some elements might by realized by the production or acquisition of assets. For example, a CG element might require that a 3D model be built. Assets are things that you produce outside of shots (say, in pre-production) or that are used across several shots. These can be things like 3D models, HDRIs, matte paintings, miniature sets or models, props, characters, etc. Elements tie Assets to Shots.

You can specify a single Asset for each Element. If you do this, then on the assets page you will see a list of all the shots where the asset is used. (If you think you need to link an Element to more than one Asset, contact me with your specific example.)

How do you distinguish between what should be an Asset and what should be an Element? Firstly, Assets represent the work needed to produce something independent of a shot, while elements represent the work needed to prepare that thing for a specific shot. For example, 3D model itself can't be composited directly -- you have to light, rig, animate and render out a scene. Secondly, Assets can be shared across multiple shots -- Elements, on the other hand, are specific to one shot.