  In this study we propose a classification scheme for the phenomenon of
subpulse drifting in pulsars. We have assembled an exhaustive list of pulsars
which exhibit subpulse drifting from previously published results as well as
recent observations using the Giant Meterwave Radio Telescope. We have
estimated detailed phase variations corresponding to the drifting features.
Based on phase behaviour the drifting population was classified into four
groups : coherent phase-modulated drifting, switching phase-modulated drifting,
diffuse phase-modulated drifting and low-mixed phase-modulated drifting. We
have re-established the previous assertion that the subpulse drifting is
primarily associated with the conal components in pulsar profile. The core
components generally do not show the drifting phenomenon. However, in core
emission of certain pulsars longer periodic fluctuations are seen, which are
similar to periodic nulling, and likely arise due to a different physical
phenomenon. In general the nature of the phase variations of the drifting
features across the pulsar profile appears to be associated with specific
pulsar profile classes, but we also find several examples that show departures
from this trend. It has also been claimed in previous works that the spin-down
energy loss is anti-correlated with the drifting periodicity. We have verified
this dependence using a larger sample of drifting measurements.
