A section of a group $G$ is a quotient of a subgroup of $G$ That is, a section of $G$ is a group of the form $H/N$ where $H$ is a subgroup of $G$ and $N$ is a normal subgroup of $H$

A group $G$ is said to be involved in a group $K$ if $G$ is isomorphic to a section of $K$

The relation `is involved in' is transitive, that is, if $G$ is involved in $K$ and $K$ is involved in $L$ then $G$ is involved in $L$

Intuitively, `$G$ is involved in $K$ means that all of the structure of $G$ can be found inside $K$