Every request sent from an X client to the X server in the X11 protocol is identified by an opcode.
Opcodes 0-127 are reserved for the core X11 protocol.   Opcodes 128-255 are used by extensions.

For the core protocol request opcodes, you can find their definitions in any of these places:
 * the `X_*` defines at the end of [[http://cgit.freedesktop.org/xorg/proto/xproto/tree/Xproto.h|/usr/include/X11/Xproto.h]]
 * the `XRequest.*` lines in [[http://cgit.freedesktop.org/xorg/lib/libX11/tree/src/XErrorDB|/usr/share/X11/XErrorDB]]
 * the `R* X11:*` lines in [[http://cgit.freedesktop.org/xorg/xserver/tree/dix/protocol.txt|/usr/lib/xorg/protocol.txt]]
 * the X protocol specs at http://www.x.org/releases/current/doc/index.html

Opcodes 128-255 are dynamically assigned to X extensions, depending on which are supported and active in your current X server version/configuration.   To see which are which in your currently running X server, run:
{{{
	xdpyinfo -queryExt | grep opcode
}}}

Each extension is assigned a single opcode from that range, also known as it's “major opcode.”
For each operation provided by that extension, typically a second byte is used as a “minor opcode.”
Minor opcodes for each extension are defined by the extension.
For definitions of those, see any of these:

 * the headers for that extension in [[http://cgit.freedesktop.org/xorg/proto/|/usr/include/X11/extensions/]]
 * the `XRequest.<extension>.*` lines in [[http://cgit.freedesktop.org/xorg/lib/libX11/tree/src/XErrorDB|/usr/share/X11/XErrorDB]]
 * the `R* <extension>:*` lines in [[http://cgit.freedesktop.org/xorg/xserver/tree/dix/protocol.txt|/usr/lib/xorg/protocol.txt]]
 * the X extension protocol specs at http://www.x.org/releases/current/doc/index.html