  We describe the digest2 software package, a fast, short-arc orbit classifier
for small Solar System bodies. The digest2 algorithm has been serving the
community for more than 13 years. The code provides a score, D2, which
represents a pseudo-probability that a tracklet belongs to a given Solar System
orbit type. digest2 is primarily used as a classifier for Near-Earth Object
(NEO) candidates, to identify those to be prioritized for follow-up
observation. We describe the historical development of digest2 and demonstrate
its use on real and synthetic data. We find that digest2 can accurately and
precisely distinguish NEOs from non- NEOs. At the time of detection, 14% of NEO
tracklets and 98.5% of non-NEOs tracklets have D2 below the critical value of
D2 = 65. 94% of our simulated NEOs achieved the maximum D2 = 100 and 99.6% of
NEOs achieved $D2 \ge 65$ at least once during the simulated 10-year timeframe.
We demonstrate that D2 varies as a function of time, rate of motion, magnitude
and sky-plane location, and show that NEOs tend to have lower D2 at low Solar
elongations close to the ecliptic. We use our findings to recommend future
development directions for the digest2 code.
