IANAL, but basically anything that negatively impacts a protected class and doesn't have a clear business purpose can be argued as discrimination. "Reasonable accommodations" without "undue hardship" are key phrases you will commonly hear as being the expectations for employers. Those accommodations can include stuff like installing ramps for an employee in a wheelchair or granting a new mother time to pump. The employee doesn't have to be fired or denied a promotion, simply making their job or life more difficult is often enough to be considered discrimination.