In brief: Combustion denotes closeness to the Sun; cazimi is an exceptionally tight conjunction treated differently in some traditions.
One angular scale, different rules
These terms begin with the elongation between a planet and the Sun. One degree contains 60 arcminutes, so 17 arcminutes is about 0.283°. Other historical authors used a full degree for the latter boundary. Always name the rule in use before labeling a chart.
What happens in the sky
A planet close to the Sun’s direction can be difficult or impossible to see against the daytime sky or bright twilight. Visibility depends on more than ecliptic longitude: latitude, horizon position, atmospheric conditions, and whether the body is east or west of the Sun matter. A longitude conjunction is not automatically a true line-of-sight overlap. The interpretive labels should not be treated as a claim that a planet’s physical properties have changed.
Why cazimi is an exception
Later traditional astrologers often treated ordinary combustion as a debility, then gave the tiny central cazimi interval a favorable exception. That sounds contradictory until the rules are understood as a historical symbolic scheme rather than a smooth physical effect. The early and later thresholds differ, so a body at 0.5° from the Sun might be “in the heart” under one author but outside a stricter 17-arcminute rule.
A practical chart check
Read the Sun and planet’s exact tropical longitudes at a named instant and take their shortest separation around the 360° circle. For example, 29° Pisces and 1° Aries differ by 2°, not 358°. Note whether the chosen combustion definition also requires the same sign. If comparing observed visibility, calculate the true sky separation and local viewing conditions separately. Use the label as a description of one tradition’s rule, not a forecast of success or harm.