Objectives are the backbone of a quest. They define the major sequence(s) of the quest, and how it progresses from the start to an eventual conclusion.
Objectives don't cover the individual actions you do to progress the quest, that is handled by child Tasks. Objectives give everything structure and group potentially multiple tasks together in an overal sub-goal. Objectives are implicitly completed when mandatory child tasks are completed.
Objectives are always sequential. Thus, a quest will typically run down the list of objectives from top to bottom; however there can be branching within that. There will always be ONE 'next objective' in a quest, although that may be comprised of multiple tasks.
As a grouping level, Objectives allow you to combine multiple tasks in different ways, depending on the Objective attributes.
Objectives are completed when the required number of mandatory tasks within them are completed. By default, that's "all mandatory tasks". However you can also tell an objective that completing just one of the mandatory tasks within it fulfils the objective, or an arbitrary number out of the ones defined.
Optional tasks never complete objectives, they're just a bonus.
An objective fails when it's impossible to complete it any more given the number of mandatory tasks left unresolved, and the number of mandatory tasks required to complete it. At the simplest level, that means if an objective requires all mandatory tasks to be completed (the default), then failing any mandatory task fails the objective.
When the current objective fails, by default the quest fails. How you handle that is up to you; that could be the end of it, or you could have other fallback quests automatically be accepted (see Auto-Accept in Quests). Or you could reset the quest back to the beginning, or even activate another quest branch within the same quest.
You can optionally set the bContinueOnFail option to true on an objective, which means that
even if the objective fails, this quest is allowed to continue to the next objective.
This could be used for tasks that are "stealth optional", in order to structure
the quest better, or you could respond to the failure by setting a Branch on the
quest, sending it down an alternative path.
One option on Objective is bSequentialTasks; whether the mandatory tasks contained within it need to be
completed in order (default true). Calls to complete sequential tasks out of order
will be ignored. This can let you simplify your world triggers so that they can
always send the message to complete a task, but it won't actually progress the
quest unless that task is the next one to be done.
Non-sequential tasks can be done in any order, so long as the Objective is active. Again, any calls to complete the task while the Objective isn't the current one in the quest will be ignored.
Optional tasks can always be completed at any time; if you need them to be ordered then use Objectives to sequence that.
Like Quests, Objectives can have identifiers which must be unique within the Quest if supplied. However, you only need to assign an Identifier to an Objective if you need one to perform queries later - e.g. you need to know whether/when a specific Objective is completed in full or failed, rather than individual tasks or the whole quest.
If you don't need to know this, you can leave the Identifier out of your definition (it will default to "None").
These are optional, and dependent on your quest UI needs, but if you want to display the collective name of the Objective to the player, or have it add extra lines to the quest description, then you can supply these in the JSON. They're only used by any quest UI you build.