Syslinux

The SYSLINUX Project covers a suite of lightweight BootLoaders, for starting up computers operating system.

Overview

the original SYSLINUX, used for booting from File Allocation Table - FAT - filesystems (usually floppy discs);
ISOLINUX, used for booting from CD-ROM ISO 9660 filesystems;
PXELINUX, used for booting from a network server using the Pre-eXecution Environment (PXE) system;
EXTLINUX, used to boot from Linux ext2/ext3 filesystems;
MEMDISK, used to boot older operating systems like MS-DOS from these media.

Debian uses syslinux for purposes :

IsoLinux :: Debian Installation CDs and DVDs (on i386, amd64, and maybe others) uses syslinux' isolinux.
PxeLinux :: Debian Installer's Netboot uses syslinux' pxelinux.

Extlinux cannot boot from ext4 filesystems with the 64bit feature enabled as documented in the syslinux wiki, see Bug 833057. The 64bit feature is enabled by default on Debian stretch and can be disabled by passing -O ^64bit to mkfs.ext4.
Booting from xfs filesystems on an MBR partition scheme does not work, see Bug 803938
Booting from xfs filesystems on a GPT partition scheme is not supported upstream, see Bug 879543 and the syslinux wiki
There is an issue with booting from btrfs, see Bug 865462