Questions about `type` in defineing problems

[Da1sypetals]

I do not exactly know how to conclude the question, so I am showing an example: For example, I want to use the Newton solver, and I have to implement Gradient and Hessian. I wonder what types can I use to be type Gradient? Am I allowed to use sparse matrices? Is this documented anywhere?

impl Gradient for Rosenbrock {
    type Gradient = /* what types are valid? */;
    // ... omitted
}

I browsed the docs and getting-started website but did not find anything related. Thanks a lot in advance❤️