  The azimuthal substructure observed in some debris disks, as exemplified by
epsilon Eridani, is usually attributed to resonances with embedded planets. In
a standard scenario, the Poynting-Robertson force, possibly enhanced by the
stellar wind drag, is responsible for the delivery of dust from outer regions
of the disk to locations of external mean-motion planetary resonances; the
captured particles then create characteristic ``clumps''. Alternatively, it has
been suggested that the observed features in systems like epsilon Eri may stem
from populations of planetesimals that have been captured in resonances with
the planet, such as Plutinos and Trojans in the solar system. A large fraction
of dust produced by these bodies would stay locked in the same resonance,
creating the dusty clumps. To investigate both scenarios and their
applicability limits for a wide range of stars, planets, disk densities, and
planetesimal families we construct simple analytic models for both scenarios.
In particular, we show that the first scenario works for disks with the pole-on
optical depths below about ~10^{-4}-10^{-5}. Above this optical depth level,
the first scenario will generate a narrow resonant ring with a hardly visible
azimuthal structure, rather than clumps. The efficiency of the second scenario
is proportional to the mass of the resonant planetesimal family, as example, a
family with a total mass of ~0.01 to 0.1 Earth masses could be sufficient to
account for the clumps of epsilon Eridani.
