Gathering Messages covers the use of the Gather button found in most of Manhattan's modules. From the student's perspective, gathering messages within the Assignments module is no different from other modules. From the teacher's perspective, the Gather command within the Assignment module is a little different.
First, you'll notice that the Gather button appears on two levels within the module. The Gather button, and associated drop-down selection list you see when you first enter the module applies to all of the assignments:

Once you select an assignment from the initial list, you'll find that the Gather command applies only to that assignment:

Another difference, from the teacher's perspective, is that the pages that show gathered messages always include:
The original assignment
All messages sent by the selected student
All messages sent by a teacher to the selected student
This makes it easier for the teacher to gather a record of all Assignment module correspondence to and from a particular student, for either an individual assignment, or for the entire course.