In general, electric (charge) radiation or magnetic (current, magnetic
moment radiation) can be classified into multipoles EL or ML
of order 
, e.g E1, E2, E3 for electric dipole,
quadrupole or octupole. The radiation field will be a sum of the
multipole contributions; however, usually one or two multipoles
dominate.
The angular distribution of gamma intensity in an appropriate coordinate
system is as 
.
A multipole of order L carries 
units of orbital angular
momentum. A photon is a vector particle and must carry at least one unit
of orbital angular momentum; thus there are no E0 and M0 radiation
fields (the latter is also forbidden by the lack of magnetic monopoles).
Note that even classically the electromagnetic field carries angular
momentum.
The parity change in a transition is essentially determined by the
behaviuor of the 
for multipole M:
for EL transitions
and for ML transitions
Conservation of angular momentum and parity lead to a set of selection rules which determine which transitions are allowed.
Successive multipoles have decay constants
Successive multipoles thus differ in decay constant by three orders of magnitude. Higher multipoles only become important when E1 transitions are forbidden by selection rules.