A hadronic event is conventionally subdivided into sets of partons that form separate colour singlets. These sets are represented by strings, that e.g. stretch from a quark end via a number of intermediate gluons to an antiquark end. Three string-mass regions may be distinguished for the hadronization.
String systems below a threshold mass are handled by the cluster machinery. In it, an attempt is first made to produce two hadrons, by having the string break in the middle by the production of a new pair, with flavours and hadron spins selected according to the normal string rules. If the sum of the hadron masses is larger than the cluster mass, repeated attempts can be made to find allowed hadrons; the default is two tries. If an allowed set is found, the angular distribution of the decay products in the cluster rest frame is picked isotropically near the threshold, but then gradually more elongated along the string direction, to provide a smooth match to the string description at larger masses. This also includes a forward-backward asymmetry, so that each hadron is preferentially in the same hemisphere as the respective original quark it inherits.
If the attempts to find two hadrons fail, one single hadron is formed from the given flavour content. The basic strategy thereafter is to exchange some minimal amount of energy and momentum between the collapsing cluster and other string pieces in the neighbourhood. The momentum transfer can be in either direction, depending on whether the hadron is lighter or heavier than the cluster it comes from. When lighter, the excess momentum is split off and put as an extra `gluon' on the nearest string piece, where `nearest' is defined by a space-time history-based distance measure. When the hadron is heavier, momentum is instead borrowed from the endpoints of the nearest string piece.
The free parameters of the model can be tuned to data, especially to the significant asymmetries observed between the production of and mesons in collisions, with hadrons that share some of the flavour content very much favoured at large in the fragmentation region [Ada93]. These spectra and asymmetries are closely related to the cluster collapse mechanism, and also to other effects of the colour topology of the event (`beam drag') [Nor98]. The most direct parameters are the choice of compensation scheme (MSTJ(16)), the number of attempts to find a kinematically valid two-body decay (MSTJ(16)) and the border between cluster and string descriptions (PARJ(32)). Also many other parameters enter the description, however, such as the effective charm mass (PMAS(4,1)), the quark constituent masses (PARF(101) - PARF(105)), the beam-remnant structure (MSTP(91) - MSTP(94) and PARP(91) - PARP(100)) and the standard string fragmentation parameters.
The cluster collapse is supposed to be a part of multiparticle production. It is not intended for exclusive production channels, and may there give quite misleading results. For instance, a quark pair produced in a collision could well be collapsed to a single if the invariant mass is small enough, even though the process in theory is forbidden by spin-parity-charge considerations. Furthermore, properties such as strong isospin are not considered in the string fragmentation picture (only its third component, i.e. flavour conservation), neither when one nor when many particles are produced. For multiparticle states this should matter little, since the isospin then will be duly randomized, but properly it would forbid the production of several one- or two-body states that currently are generated.