  The Galactic microquasar GRS 1915+105 exhibits at least sixteen types of
variability classes. Transitions from one class to another could take place in
a matter of hours. In some of the classes, the spectral state transitions
(burst-off to burst-on and vice versa) were found to take place in a matter of
few to few tens of seconds. In the literature, there is no attempt to
understand in which order these classes were exhibited. Since the observation
was not continuous, the appearances of these classes seem to be in random
order. Our goal is to find a natural sequence of these classes and compare with
the existing observations. We also wish to present a physical interpretation of
the sequence so obtained using two component advective flow model of black hole
accretion. In the present paper, we compute the ratios of the power-law photons
and the black body photons in the spectrum of each class and call these ratios
as the `Comptonizing efficiency'(CE). We sequence the classes from the low to
the high value of CE. We clearly find that each variability class could be
characterized by a unique average Comptonizing efficiency. The sequence of the
classes based on this parameter seem to be corroborated by a handful of the
observed transitions caught by Rossi X-ray timing explorer and the Indian
payload Indian X-ray Astronomy Experiment and we believe that future
observation of the object would show that the transitions can only take place
between consecutive classes in this sequence. Since the power-law photons are
produced by inverse Comptonization of the intercepted soft-photons from the
Keplerian disk, a change in CE actually corresponds to a change in geometry of
the compton cloud. Thus we claim that the size of the Compton cloud gradually
rises from very soft class to the very hard class.
