  We analyze systems of clusters and interacting upon colliding---a collision
between two clusters may lead to merging or fragmentation---and we also
investigate the influence of additional spontaneous fragmentation events. We
consider both closed systems in which the total mass remains constant and open
systems driven by a source of small-mass clusters. In closed systems, the size
distribution of aggregates approaches a steady state. For these systems the
relaxation time and the steady state distribution are determined mostly by
spontaneous fragmentation while collisional fragmentation plays a minor role.
For open systems, in contrast, the collisional fragmentation dominates. In this
case, the system relaxes to a quasi-stationary state where cluster densities
linearly grow with time, while the functional form of the cluster size
distribution persists and coincides with the steady state size distribution of
a system which has the same aggregation and fragmentation rates and only
collisional fragmentation.
