  Sub-subgiants are a new class of stars that are optically redder than normal
main-sequence stars and fainter than normal subgiant stars. Sub-subgiants, and
the possibly related red stragglers (which fall to the red of the giant
branch), occupy a region of the color-magnitude diagram that is predicted to be
devoid of stars by standard stellar evolution theory. In previous papers we
presented the observed demographics of these sources and defined possible
theoretical formation channels through isolated binary evolution, the rapid
stripping of a subgiant's envelope, and stellar collisions. Sub-subgiants offer
key tests for single- and binary-star evolution and stellar collision models.
In this paper, we synthesize these findings to discuss the formation
frequencies through each of these channels. The empirical data, our analytic
formation rate calculations, and analyses of sub-subgiants in a large grid of
Monte Carlo globular cluster models suggest that the binary evolution channels
may be the most prevalent, though all channels appear to be viable routes to
sub-subgiant creation (especially in higher-mass globular clusters). Multiple
formation channels may operate simultaneously to produce the observed
sub-subgiant population. Finally, many of these formation pathways can produce
stars in both the sub-subgiant and red straggler (and blue straggler) regions
of the color-magnitude diagram, in some cases as different stages along the
same evolutionary sequence.
