A Milestone can have many Challenges. Challenges are just extra Objectives that provide a fun way to mix-up play and provide extra rewards.
Type: object
Object Properties
Mapped to Definition
The objective for the Challenge, which should have human-readable data about what needs to be done to accomplish the objective. Use this hash to look up the DestinyObjectiveDefinition.
Type: uint32
Mapped to Manifest Database Definition: Destiny.Definitions.DestinyObjectiveDefinition
Nullable Mapped to Definition
IF the Objective is related to a specific Activity, this will be that activity's hash. Use it to look up the DestinyActivityDefinition for additional data to show.
Type: uint32
Mapped to Manifest Database Definition: Destiny.Definitions.DestinyActivityDefinition