Let $X$ be a K3 surface. Let $E$ be a semistable rank 3 vector bundle. Now suppose $0 = E_0\subset E_1\cdots\subset E_s=E$ be the Harder-Narasimhan filtration. Suppose $E_1$ is $\mu$-stable and rank $E_1$ is 2. Then the paper says that $E$ sits in the following short exact sequence :

$0\longrightarrow E_1\longrightarrow E\longrightarrow N\otimes I_\xi\longrightarrow 0$ where $N$ is a line bundle and $I_\xi$ is the ideal sheaf of a 0-dimensional subscheme $\xi\subset X$.

How do we get this exact sequence? What are $N$ and $\xi$ exactly? Any help will be appreciated! Thanks in advance!