Conditions are checked in order. Once one matches, the rest are skipped. This matters when conditions overlap:
# Wrong order
if score >= 70:
grade = "C"
elif score >= 80:
grade = "B" # Never reached!
elif score >= 90:
grade = "A" # Never reached!
A score of matches score >= 70 first, so it gets "C" instead of "A". Always check the most specific condition first.
# Correct order
if score >= 90:
grade = "A"
elif score >= 80:
grade = "B"
elif score >= 70:
grade = "C"