over $K$ is a pair $(C,\lambda)$ where

1.  $C$ is a geometrically reduced, geometrically connected projective curve of genus 0 over $K$ with at worst nodal singularities, and

2.  $\lambda\colon I\into C^{\mathop{\rm sm}}(K),\,\,i\mapsto\lambda_i,$ where $C^{\mathop{\rm sm}}$ denotes the smooth locus of $C$, is an injective map such that for each irreducible component $E\subset C$, the number of marked points $\lambda_i$ on $E$ plus the number of singular points on $E$ is $\geq 3$.

We call the map $\lambda$ an *$I$-marking* of $C$. Since we never consider curves of higher genus, we use the phrase “stable $I$-marked curve” in place of “stable $I$-marked curve of genus 0” from now on. We visualize such curves as trees of copies of ${{\mathbb{P}}}^1_K$. Stable $I$-marked curves and their generalization to arbitrary base schemes are discussed in detail in Section \[stablemarkedcurves\].

Let $V:=\bigl(t^{-1}{{\mathbb{F}}}_q[t]/{{\mathbb{F}}}_q[t]\bigr)^n$. A Drinfeld ${{\mathbb{F}}}_q[t]$-module of rank $n$ and level $(t)$ over a field extension $K$ of ${{\mathbb{F}}}_q(t)$ may be viewed as a pair $(\psi,\lambda)$ consisting of a homomorphism $$\begin{aligned}
\psi\colon{{\mathbb{F}}}_q[t]&\longto&\End_K({{\mathbb{G}}}_{a})\cong K[\tau]\\
a&\mapsto& \psi_{a}\end{aligned}$$ along with an injective ${{\mathbb{F}}}_q$-linear map $$\label{lvlstr}\lambda\colon V\stackrel\sim\to\ker(\psi_t)(K)\subset{{\mathbb{G}}}_a(K)=(K,+).$$ In fact $\psi$ is determined by $\psi_t$, which is in turn determined by its zeros and the condition that $\psi_{t,0}=t$, so the 