What is the n-concavification of a Banach space?

Question

I'm reading this paper about polynomials in Banach spaces and the authors use the notion of the n-concavification of a Banach space $X$

It is the first time that I encounter this concept. What is it? How is it defined?

A quick Google search pointed me to results that seemed unclear to me at first.

I would like if anyone could point me to a reference to understand what it is.

Answer 1

I found the answer (in the same paper, I had overlooked that section when glancing through it). The n-concavification of $X$ is:

$$X_{(n)}= \{f^n: f \in X \}$$