Asus RS300-E8-RS4 - Manual (Page 48)

Chapter 3: Installation options 3-4
Do not install the rail kit in the following situation:
DO NOT place the rail hook on a thick lip of
the mounting hole.
DO NOT install the rail to the outer side of
the server rack.
7. When mounting the server to the rack,
ensure to include the side knots on the
two sides of the server in the rack rail
holders, as shown in the right figure.