When a player chooses the order in which they complete tasks it provides a feeling of autonomy. It gives the player a feeling of importance and control as they are essentially masters of what happens when.
In an undirected graph the users are allowed to traverse from any node to any other node, with no restrictions on the priority of nodes.
This is the minimap of GTA 4. As you can see there are various icons on the map. The letters F and J on the map represent the missions the player can choose to complete. The player is required to complete all the missions to progress the game. But as you can see the game does not force the player into completing these mission in any particular order. The lens of order is illustrated well via this.
Giving the players control over what goals they should achieve when, essentially means they dictate the narrative. If your narrative is extremely strict then do not use this lens. To successfully be able to use this lens your narrative must be flexible. So event are going to occur in orders you didn't expect them to, and this should not change the experience of the player drastically.
A common thing to do when allowing players to choose the order in which goals are completed is to chain certain goals so as to plan for a certain occurrence. This would mean incorporation of the rule that to unlock certain goals, certain other goals must be achieved.
A simple guideline to use this lens is :-
Motivations :- Autonomy
Components :- Unrestricted Ordering of Goals