When you create an assignment, you can choose to release it to your students individually or in groups, depending on your needs. You can also release an assignment to all students, but send additional instructions to a specific group. You will have access to the release options on the Create/Edit Assignment screen until a student submits an attempt for the assignment.
After you have created an assignment, however, you can still set release criteria that go beyond students or groups of students.

To edit a release criterion you have already created:
In the Set Release Criteria screen, you can narrow down the release criteria for an assignment by creating more criteria and subordinating them. This creates an expression that regulates the application of your criteria.
To see an example of advanced release criteria being applied:
You can also access this topic through the Help Contents navigation (left pane) by expanding Using Blackboard Learning System Features and Tools > Selective Release > Setting Release Criteria for Items.