The Release Deadline button changes the deadline for the active assignment phase from the previously established deadline date and time to the instant you click that button.
For example, if you press the Release Submission Deadline button, the assignment will automatically end the Submission phase and move the assignment into the Review phase and all subsequent uploads will be late submissions. If you press the Release Review Deadline button, the assignment will automatically end the Review phase and move the assignment into the Feedback phase.
Please note that the Release Deadline button does not affect the grace period settings. If you'd like to remove a grace period, please change this setting on the Assignment Settings page.
Typically, the Release Deadline buttons are used for in-class assignments during which students will move through all 3 phases of a Peerceptiv assignment (submission, review, feedback) in a single "live" class session.