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[Guest_BruceMoomaw_*]

http://mepag.jpl.nasa.gov/reports/Draft_SR-SAG_v25.doc

Ironically, because I don't have the "graphics conversion filter" for Wordpad, I can't read their definition directly -- but Table 9.1, near the end of the article, makes it clear:
____________________________________________

Classification of Martian environments by their potential to exceed the threshold conditions in temperature and water activity for microbial propagation (within the boundary conditions of the analysis):

A. Observed features for which there is a significant (but still unknown) probability of association with modern liquid water:

1 -- Recent gullies and gully-forming regions
2 -- Mid-latitude “pasted-on” terrain

B. Observed features for which there is a low, but non-zero, probability of a relationship to modern liquid water:

3 -- Low-latitude slope streaks
4 -- Low-latitude features hypothesized to be glaciers

C. Not known to exist, but if examples could be found, would have a high probability of association with modern liquid water:

5 -- Volcanic environments young enough to retain heat
6 -- Impact environments young enough and large enough to retain heat
7 -- Modern outflow channels

"Of these, only the mid-latitude pasted-on terrain and the gully-forming regions are thought to be of high enough concern for exceeding the threshold conditions during the next 100 years that the SR-SAG believes they should be treated as special regions for the purposes of planetary protection... The gullies and pasted-on terrain both occur at middle latitudes."