If a task is created while viewing 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.
Tasks that are created while viewing the Tasks tab (not a plan) are designated as non-geo-tagged tasks, and have a square icon. These signify general work or reminders 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.