Synology uses a custom fork of the Linux kernel to power DSM. It has a lot of great improvements. Some of them deal with being useful for any NAS while others are feature-flags for their hardware. This document lists and describes more useful/interesting flags present in their kernel(s). Keep in mind this is not a definitive list.
See kernel/syno_bootargs.c for a complete list of options for a given kernel.
/* More stuff will go here */
placeholder