The "Assignable" option unlocks the assignment workflow features of WorkflowFirst. This is the central means through which the workflow functions.

Once a type is set to Assignable, that record can then be assigned to one or more users. Once a record is assigned:

1. An email will be sent out to the user to tell them the record is assigned to them.
2. The record will be highlighted in red to the assignee, as will the parent records, to alert the user to the assignment.
3. The tab itself will have a red "badge" count of all the assigned records in that tab. 
4. An entry will also appear for that record in the Dashboard tab.
5. Reminders can be set with specific dates, which will appear in the calendar on the dashboard, and email to the user when the date comes about.
6. The record can be reassigned to a different user.

All of these features are enabled automatically as soon as the record is set to being assigned.

The functionality is often invoked through the workflow:AddAssignment script call, but is usually automatically made available as soon as you add a workflow to your form. All of the workflow functionality revolved around this assignment feature.