What do slanted square brackets mean in medical coding?

Slanted brackets are used in the Index to Diseases of the ICD-9-CM coding book and they identify manifestation codes. A manifestation is a condition that occurs as the result of another condition, and manifestation codes are ALWAYS reported as secondary codes. In other Words, when you see a code in slanted brackets (listed after a primary condition code), you MUST report and sequence it as the secondary code.