This shower reconstruction is based on the information from pandora. It comes with the outputs of pandora in larsoft/uboonecode v06_26_00 and later releases.
It is produced in the producers pandoraCosmic and pandoraNu. Regarding the shower related objects, those producers create recob::PFParticles, recob::Showers, recob::PCAxis, and association between those three. Currently they are 1-to-1.
If you want to access the clusters associated to the showers, you have to find the associated PFParticle first, and access the clusters associated to the PFParticle.
Currently the shower-related variables are not in the analysis tree, and we recommend you to use gallery to access the information.