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.
Mapped to DestinyActivityDefinition in the manifest.
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.
Mapped to DestinyObjectiveDefinition in the manifest.
A Milestone can have many Challenges. Challenges are just extra Objectives that provide a fun way to mix-up play and provide extra rewards.