How to format a USB flash drive with High Sierra?

Choose the format that is most suitable.

Mac OS Extended (Journaled) is most compatible with older Mac OS's.

APFS is the "new" file system which should be optimized for SSDs and I assume flash memory.

If you need it encrypted, you can format either format with encryption.

You can also use ExFAT which will work on both Windows and Mac OS.

You may need to select the Device to erase as opposed to the volume (partition).

In Disk Utility, above the drive list, click the View button and choose Show all devices.

Then, select the usb device as opposed to the volume.

There really isn't much difference with respect to a usb drive.

Only High Sierra and Mojave can use an APFS formatted drive.

I only use usb flash memory for SneakerNet transfers, so the portability of the file system would be important to me.

If you are not transporting files with the device, either format is sufficient.

FWIW, Chromebooks can read and write to Windows formatted drives; MSDOS, FAT32, exFAT, and NTFS. It can read Mac HFS+ drives but not write to them. If you want something compatible with Macs and Chromebooks, exFAT would be the best choice.

You're welcome.