  Hellsten \cite{MR2026390} gave a characterization of $\Pi^1_n$-indescribable
subsets of a $\Pi^1_n$-indescribable cardinal in terms of a natural filter
base: when $\kappa$ is a $\Pi^1_n$-indescribable cardinal, a set
$S\subseteq\kappa$ is $\Pi^1_n$-indescribable if and only if $S\cap
C\neq\emptyset$ for every $n$-club $C\subseteq \kappa$. We generalize
Hellsten's characterization to $\Pi^1_n$-indescribable subsets of
$P_\kappa\lambda$, which were first defined by Baumgartner. After showing that
under reasonable assumptions the $\Pi^1_0$-indescribability ideal on
$P_\kappa\lambda$ equals the minimal \emph{strongly} normal ideal
$\text{NSS}_{\kappa,\lambda}$ on $P_\kappa\lambda$, and is not equal to
$\text{NS}_{\kappa,\lambda}$ as may be expected, we formulate a notion of
$n$-club subset of $P_\kappa\lambda$ and prove that a set $S\subseteq
P_\kappa\lambda$ is $\Pi^1_n$-indescribable if and only if $S\cap
C\neq\emptyset$ for every $n$-club $C\subseteq P_\kappa\lambda$. We also prove
that elementary embeddings considered by Schanker \cite{MR2989393} witnessing
\emph{near supercompactness} lead to the definition of a normal ideal on
$P_\kappa\lambda$, and indeed, this ideal is equal to Baumgartner's ideal of
non--$\Pi^1_1$-indescribable subsets of $P_\kappa\lambda$. Additionally, as
applications of these results we answer a question of Cox-L\"ucke
\cite{MR3620068} about $\mathcal{F}$-layered posets, provide a characterization
of $\Pi^m_n$-indescribable subsets of $P_\kappa\lambda$ in terms of generic
elementary embeddings, prove several results involving a two-cardinal weakly
compact diamond principle and observe that a result of Pereira \cite{MR3640048}
yeilds the consistency of the existence of a $(\kappa,\kappa^+)$-semimorasses
$\mu\subseteq P_\kappa\kappa^+$ which is $\Pi^1_n$-indescribable for all
$n<\omega$.
