Her's what I can tell you, having been the pee test supervisor for a 121 company. Keep in mind the regs allow some leeway in the process, but most places farm this job out to cut costs and keep their distance from the process for legal reasons.
Pre employment: Everybody takes the IQ test, period.
Random: Company seperates names from SSNs, sends list of SSNs to vendor. Vendor draws SSNs out of hat at least 12 times per year, but even that draw date & frequency is random. SSNs go to company pee person, who matches SSN to person and checks schedules for the ambush date/time. Pee person has 30 days to ambush you. Once you are snagged, you have 2 hours to report for testing. FAA sets percentages of required personnel (safety sensitive folks) and changes the percentages occasionally based on pass/fail rate and politics. Vendor sends slightly more than required rate to account for vacations, LOAs, terminations. Company is responsible for keeping list current to vendor. Company is also supposed to ambush randomly, not just at begining or end of trips (yeah right, I always got it at the end of a 14 hr duty day!).
Probable Cause: This is serious. Implies suspicion of current drug/alcohol use, not "I knew Fred in college 20 years ago, he was a pot head". Requires TWO drug policy trained supervisors to agree on evidence to send you testing. Your record will show test type as probable cause even if you are clean, so avoid this at all costs.
They have to tell you the test type in writing on the form.
Glad I could help. If you get randomed all the time change your SSN!