If a task is created directly on a plan, that task is considered to be a geo-tagged task (one that is specific to that location on the plan), and is indicated by a marker pin icon.
Tasks created while viewing a plan, but not placed in a specific spot, are given a diamond icon. These tasks are a great way to describe widespread issues or comments which are relevant for the entire drawing.
Tasks that are created while viewing the Tasks tab (i.e. are not affiliated with a plan) will have a square icon. These signify general work or milestones for the overall project, and are not associated with a specific location. These tasks can be later moved onto plan.
For more information on geo vs non-geo-tagged tasks, please visit the Task Tab article.
Task is associated with a specific location on plan.
Task is associated with a plan, but not a specific location.
Task is not associated with a plan.