Most of it is covered by the following articles, etc. SLOG is not a write cache. You only need it for sync writes. So databases, Block storage (iSCSI, zvols for VMs), and NFS. Larger SLOG devices can be underprovisioned and should have PLP (power loss prevention) and proper write endurance. Should be as fast or faster than your pool, in general.
BASICS
Special VDEV (sVDEV) Planning, Sizing, and Considerations
