Bonk uses the Transform2D struct which is taken from the MoonTools.Core.Structs package. By extension it also uses the Position2D struct taken from the same package.
A Transform2D is basically a way to store information about an object’s location, rotation, and scale in 2-dimensional space. Transforms use matrix math to perform all of these operations at once, making them very fast.
For memory performance purposes, shapes are implemented as structs, meaning they are value types. This means that it is much easier to deal with collision using a transform applied to a shape, rather than containing a transform within the shape.
It is also often the case that you will have a shape attached to a game object that is offset from the object by a certain amount. A Transform2D can represent this information as well, since Tranform2Ds can be composed together. As you can see, a Transform2D provides a generic structure for dealing with many scenarios involving 2D space.