Documents needed at the airport

I need some clarification. A a Permanente, which part of the form must be kept prior to leaving the airport. Bottom or top portion?

They (the airline) take the bottom. You take the top and use that when you return into Mexico. Do not fill out another FMM form. Give customs the top, your passport and your Permanate card when you arrive back.

Also be sure to check "other" as your reason for travelling, check off the "residente permanente" boxes (top and bottom) and write "Residente Permanente" across the top of the form.