The "Items provided" in the reward. This is almost always a pointer to a DestinyInventoryItemDefintion for an item that you can't actually earn in-game, but that has name/description/icon information for the vague concept of the rewards you will receive. This is because the actual reward generation is non- deterministic and extremely complicated, so the best the game can do is tell you what you'll get in vague terms. And so too shall we.
Interesting trivia: you actually do earn these items when you complete the activity. They go into a single-slot bucket on your profile, which is how you see the pop-ups of these rewards when you complete an activity that match these " dummy" items. You can even see them if you look at the last one you earned in your profile-level inventory through the BNet API! Who said reading documentation is a waste of time?
The header for the reward set, if any.
Activities can refer to one or more sets of tooltip-friendly reward data. These are the definitions for those tooltip friendly rewards.