In this case, you consider $D$ a divisor (with normal crossing) on a nonsingular variety $X$ of dimension $n$. So, with this notation, the singular set of $D$ is an analitic subvariety of $X$ of codimension $n-2$. More precisely, if you write $D = \cup_{i} D_{i}$ with $D_{i}$ its irreducible components, the singular set of $D$ is given by

$ sing(D) = \bigcup_{i \neq j} D_{i} \cap D_{j}.$

In this case, you consider $D$ a divisor (with normal crossing) on a nonsingular variety $X$ of dimension $n$. So, with this notation, the singular set of $D$ is an analitic subvariety of $X$ of codimension $2$. More precisely, if you write $D = \cup_{i} D_{i}$ with $D_{i}$ its irreducible components, the singular set of $D$ is given by

$ sing(D) = \bigcup_{i \neq j} D_{i} \cap D_{j}.$