Ned just attended an orientation session for new employees. The employee benefits director spoke for an hour about the various employee benefits the company offered and the provisions of these benefits. During the presentation, the employee benefits director discussed (1) probationary periods, (2) eligibility periods (open enrollment), and (3) elimination (waiting) periods. Ned is confused about these terms and has asked you to explain the meaning and importance of each period. How would you describe these periods to Ned?