If the features (points, lines, or polygons) in the datalayer to be symbolized overlap, it may be desirable to specify the order of each type of feature. Otherwise the features are drawn according to the index which is not readily apparent. The order of specification in the SLD does not support the actual order of drawing unless each <Rule> tag is surrounded by its own <FeatureTypeStyle> tag.
Boundless also wrote up a description of deterministic rendering order in SLDs.
Example of features drawn without order specified: ( no <FeatureTypeStyle> tags used around <Rule> tags)
SLD Result: (solid orange sometimes draws on top of blue dots or blue slash)
Example of features drawn with order specified ( <FeatureTypeStyle> tag used around <Rule> tags).
SLD below - the goal was to "Low Confidence" polygons underneath symbolized on solid orange, then "Inundated Areas" next with a dot pattern and finally "Unconnected Low-Lying areas" on top with a slash pattern.
As shown in MORIS: