Global statuses can be selected for each milestone within a workflow.
If you imagine having multiple workflows, with different milestones and stages, then imagine having thousands of cases live within Fluid, it would be hard to keep track of your overall progress in one unified approach. This is where Global Statuses are a really handy feature. You can set up a more general status for the progress of a case that can be shared across all of your workflows. For instance, you might have lots of different cases which all go through paraplanning. The paraplanning stages might be spread across multiple milestones and stages, but give each milestone the 'Paraplanning' global status, and from your cases table you can easily see or filter all of the cases which are in paraplanning.
You can set up Global Statuses from your settings page, within the Case Statuses tab. Each Global Status can a colour assigned to it. You can add as many statuses as you see fit.
If no status is set for a milestone, the milestone itself will be visible from the cases table without a specific colour set.