How to overload similar_type for FieldVector?

I am really confused by the documentation on what I need to do to get FieldVector to work correctly regarding similar and similar_type.

I have:

struct CylindricalStress{T<:Number} <: FieldVector{3,T}
    r::T
    θ::T
    z::T
end

Somewhere later my type is reverted to 3-element StaticArraysCore.MVector{3, Float64} with indices SOneTo(3).

The similar_type documentation shows four "relavent" methods but then says to "only overload the last method"? I don't know what any of those similar_type methods are doing or what I should be doing with them. What is A? What if I don't know my ElType? I don't want to change my element type or size, so why do I need to use the last method? Why do none of the options have <: FieldVector? I'm totally lost.

similar_type(::Type{A}) where {A <: StaticArray} # defaults to A
similar_type(::Type{A}, ::Type{ElType}) where {A <: StaticArray, ElType} # Change element type
similar_type(::Type{A}, size::Size) where {A <: AbstractArray} # Change size
similar_type(::Type{A}, ::Type{ElType}, size::Size) where {A <: AbstractArray, ElType} # Change both

Start with the similar_type documentation and the FieldVector entry point mentioned in the issue. Trace the four documented method forms and how they relate to the reported conversion to MVector. Done means the documentation clearly explains the relevant method, its parameters, and how a FieldVector user keeps the intended type.