SNAP consists of multiple modules and can be extended by further more. Many of the modules are grouped in so called 'clusters'. Simply spoken just a bunch of modules. The toolboxes and SNAP it self are clusters. The clusters are located in the installation directory. The folders snap, s1tbx, s2tbx and s3tbx are all clusters.
A SNAP installation can be extended by additional clusters. A cluster is created during the build process of project. More on this at the end
Adding a cluster to SNAP Desktop
In order to let SNAP desktop start with an additional cluster you need to edit the snap.conf file.
Within this file you find the property extra_clusters
# Extra cluster paths separated by semicolon ';' #extra_clusters=""
Add the path to the cluster directory and remove the '#' at the line start.
extra_clusters="D:\Projects\myProject\target\netbeans_clusters\extra"
If the path ends with 'extra' depends on the project configuration it can have some other name.