I have a SDXC UHS card. I am trying to determine what I should use before I remove the card from it's slot.
This option will "literally" unmount the device/partition and it applies normally (In Nautilus) to hard drives, be them, internal or external. Is not common to see it for Flash Drives / Pen Drives / Thumb Drives / SD Card / Digital Camera and any other device that needs to sync before powering down.
This is not the recommended way of doing it when addressing a device that needs to power down first if your intention is to disconnect the device afterwards, as the device will still show in the Nautilus panel..
Have in mind that unmount gives you the ability to mount the device again (since it did not power down or actually eject the device from the system).
Eject / Eject Button
Somehow similar to Unmount, it will unmount the device/partition. The problem is that this option should only be present for Optical Devices (Devices that do not need to sync data before disconnecting) since this actually EJECTS the device. I mean literally ejects or opens your CD Drive/DVD Drive Device so you can take the CD/DVD out of it, physically speaking.
You will not be able to mount the device again, except if you insert the CD/DVD back in (With Optical devices) or remove and reinsert the USB Drive again (For Flash Drives and other similar devices). Since this option also appears for USB Drives, and CD/DVD Drives behave differently than USB Drives (Hard Drives and Pen Drives in this case, Optical devices are omitted), ejecting a not properly synced USB Drive (Flash Drive or Hard Drive) could cause problems.