Code  
Description
Milling
( M )
Turning
( T )
Corollary info
M00 Compulsory stop M T Non-optional—machine will always stop upon reaching M00 in the program execution.
M01 Optional stop M T Machine will only stop at M01 if operator has pushed the optional stop button.
M02 End of program M T No return to program top; may or may not reset register values.
M03 Spindle on (clockwise rotation) M T The speed of the spindle is determined by the address S, in either revolutions per minute (G97 mode; default) or surface feet per minute or [surface] meters per minute (G96 mode [CSS] under either G20 or G21). Theright-hand rule can be used to determine which direction is clockwise and which direction is counter-clockwise.Right-hand-helix screws moving in the tightening direction (and right-hand-helix flutes spinning in the cutting direction) are defined as moving in the M03 direction, and are labeled “clockwise” by convention. The M03 direction is always M03 regardless of local vantage point and local CW/CCW distinction.
M04 Spindle on (counterclockwise rotation) M T See comment above at M03.
M05 Spindle stop M T
M06 Automatic tool change (ATC) M T (some-times) Many lathes do not use M06 because the T address itself indexes the turret.Programming on any particular machine tool requires knowing which method that machine uses. To understand how the T address works and how it interacts (or not) with M06, one must study the various methods, such as lathe turret programming, ATC fixed tool selection, ATC random memory tool selection, the concept of “next tool waiting”, and empty tools. These concepts are taught in textbooks such as Smid,[1] and online multimedia (videos, simulators, etc.); all of these teaching resources are usually paywalled to pay back the costs of their development. They are used in training classes for operators, both on-site and remotely (e.g., Tooling University).
M07 Coolant on (mist) M T
M08 Coolant on (flood) M T
M09 Coolant off M T
M10 Pallet clamp on M For machining centers with pallet changers
M11 Pallet clamp off M For machining centers with pallet changers
M13 Spindle on (clockwise rotation) and coolant on (flood) M This one M-code does the work of both M03 and M08. It is not unusual for specific machine models to have such combined commands, which make for shorter, more quickly written programs.
M19 Spindle orientation M T Spindle orientation is more often called within cycles (automatically) or during setup (manually), but it is also available under program control via M19. The abbreviation OSS (oriented spindle stop) may be seen in reference to an oriented stop within cycles.The relevance of spindle orientation has increased as technology has advanced. Although 4- and 5-axis contour milling and CNC single-pointing have depended on spindle position encoders for decades, before the advent of widespread live tooling and mill-turn/turn-mill systems, it was seldom relevant in “regular” (non-“special”) machining for the operator (as opposed to the machine) to know the angular orientation of a spindle except for within a few restricted contexts (such as tool change, or G76 fine boring cycles with choreographed tool retraction). Most milling of features indexed around a turned workpiece was accomplished with separate operations on indexing head setups; in a sense, indexing heads were invented as separate pieces of equipment, to be used in separate operations, which could provide precise spindle orientation in a world where it otherwise mostly didn’t exist (and didn’t need to). But as CAD/CAM and multiaxis CNC machining with multiple rotary-cutter axes becomes the norm, even for “regular” (non-“special”) applications, machinists now frequently care about stepping just about any spindle through its 360° with precision.
M21 Mirror, X-axis M
M21 Tailstock forward T
M22 Mirror, Y-axis M
M22 Tailstock backward T
M23 Mirror OFF M
M23 Thread gradual pullout ON T
M24 Thread gradual pullout OFF T
M30 End of program with return to program top M T
M41 Gear select – gear 1 T
M42 Gear select – gear 2 T
M43 Gear select – gear 3 T
M44 Gear select – gear 4 T
M48 Feedrate override allowed M T
M49 Feedrate override NOT allowed M T This rule is also called (automatically) within tapping cycles or single-point threading cycles, where feed is precisely correlated to speed. Same with spindle speed override and feed hold button.
M52 Unload Last tool from spindle M T Also empty spindle.
M60 Automatic pallet change (APC) M For machining centers with pallet changers
M98 Subprogram call M T Takes an address P to specify which subprogram to call, for example, “M98 P8979” calls subprogram O8979.
M99 Subprogram end M T Usually placed at end of subprogram, where it returns execution control to the main program. The default is that control returns to the block following the M98 call in the main program. Return to a different block number can be specified by a P address. M99 can also be used in main program with block skip for endless loop of main program on bar work on lathes (until operator toggles block skip).

From: Wikipedia | Sources: SmidGreen et al.