What does @_## mean in @code_warntype (and other questions)?

General Usage
7

For some things, I preallocate the size and pass this as an argument to create the object, for example v
For others, I define it within the function. Which one is correct?

const num_nutrient = 2

function create_vectors()
    v = zeros(num_nutrient)
    v::Vector{Float64} = create_v(num_nutrient,v)
end

function create_v(num_nutrient,v)
    v[1] = 1.0
    v[2] = 0.5
    return v
end

Is this the correct way?