You can select among GUID Partition Map, Master Boot Partition, and Apple Partition Map. For all modern Macs, GUID Partition Map is the only choice and it nearly never needs to be changed; Master Boot Partition is useful for cross-platform drives that can be mounted in Windows and in macOS, Apple Partition Map dates back to PowerPC Macs.

The format describes the way divisions in the disk are organized to read as files and directories by an operating system. For drives only mounted on Macs, APFS is the current best choice, but is only backwards compatible a few versions of macOS; for maximum compatibility, pick HFS+, which is labelled "Mac OS Extended (Journaled)" in Disk Utility.

Format always appears when you select an item and click Erase, but Scheme never for you. Why is that? Because you’re displaying only volumes, not drives; nor are they containers (for APFS formatted drives and partitions).

From the View menu in the upper-left corner, select Show All Devices, and you will see the hierarchy of drive > volume (most formats) or drive > container > volume (APFS). Now, you can select a drive, click Erase, and view the Scheme options for the drive as well as the default format type applied when the drive has completed applying the new scheme.