Reagarding I-9 verification

When Employer's use i-9 verification, do they only see whether the potential employee is eligible to work or do they also see the expiration date of eligibility for alien-workers like us approved with TPS. I am sure work authorization should be the sole factor for eligibilty and it's illegal to discriminate agains legal workers regardless of immigration status. What I am not sure is how the I-9 works for employers and what information they can access regarding the potential employee.