It refers to the type of vertexed particle it is (or unvertexed), and what constraint it has (unconstrained, target, or beamspot).

e.g. bsc_V0Candidate, uc_MollerCandidates, fs_particles (individual particles with no vertexing), or mc_particles.

The list of particle types found in a dst event can be seen in the source code:

/u/group/hps/hps_soft/hps-dst/src/hps_event/HpsEvent.cxx

An example of how to declare a particular type (as of the current dst-maker version) is:

const HpsParticle::ParticleType uc_moller = HpsParticle::UC_MOLLER_CANDIDATE;

then

int n_uc_mollers = ev1->getNumberOfParticles(uc_moller);

HpsParticle *Moller

loop over events {

        }

}

      V0 particles work the same way. It's a little strange, because a "vertexed particle" (e.g. "Moller" above) is actually an array of two HpsParticles. So you get things like the mass and vertex from the "Moller", but almost everything else (momentum, etc.) from the two particles you extract from it.

      I hope this is what you were asking...