Define $w_n:=\sum_{d|n} dX_d^{n/d} (\in \mathbb{Z}[X_1,X_2,...])$ for each $n\in\mathbb{Z}^+$. These are called the $n$-th Witt polynomials.
Let $P\subset \mathbb{Z}^+$ be a nonempty set such that if $n\in P$, then all proper divisors of $n$ are also in $P$.
Let $R$ be a commutativ ring and define the set $W_P(R):=R^P$.
It is written in my context that $w_n$ $(n\in P)$ can be considered as a set-theoretic map $W_P(R)\rightarrow R$, and for $x\in W_P(R)$, $w_n(x)$ is called the ghost component of $x$.
I do not understand this last paragraph. How do we consider $w_n$ as a set theoretic map? Does this mean that we consider $w_n$ as the function $x\mapsto \sum_{d|n} dx_d^{n/d}$, which is merely the evaluation map?