Upon installing High Sierra, if the boot volume is a SSD it will be automatically converted to the APFS format.  If the boot volume is a HDD or Apple Fusion Drive, it will not be converted and remain formatted as HFS+ (Mac OS Extended).

SSD is any solid-state drive (a.k.a. flash based storage), HDD is a traditional spinning hard drive, and Apple Fusion Drives are a combination of the two.

This means that depending on the Mac’s hardware configuration, High Sierra may be running on either APFS or HFS+.

There is no way to opt-out of 10.13’s APFS conversion of SSD boot volumes, and both Apple OEM and internal after-market SSDs are treated the same.  If everything goes smoothly, no data is lost during this conversion.

If the Mac has additional internal or external volumes, they will not be converted to APFS during the High Sierra install, even if they are SSD.

Apple has also committed to making APFS work with Fusion Drives in the near future.  When this happens a High Sierra update will likely also convert them.

HDDs can manually be converted to APFS, but beta testers have reported poor speed and boot volumes stability.  It is unknown if Apple will be able to address this in the future.