In response to the post above. I have limited experience using the "Multiple Exposure" feature but, as I remember, I could not find any setting that was not as slow as molasses, so I never considered using it with the Godox Mask feature. Maybe I over looked something and I may need to go back and take another look at the "Multiple Exposure" feature when I have time. What I have found that works best to date, is the method I have outlined below for capturing the images and then post processing the images in Photoshop. The method below works for both masking setups as well as for controlling flash/ambient exposures and flash contrast. If someone, with more experience in using the "Multiple Exposure" feature, can find some settings to utilize the MASK feature, please share.
The following method works, when using the Godox MASK feature with the 5DmIII and Godox XT16 transmitter combo, when precise control of shot count is required in "continuous H" Drive mode.
You need to use the non TTL XT16 transmitter so the camera will not disable AEB Auto Exposure Bracketing. The X1c and XT32 transmitters will disable AEB which is required for precise shot count control. You can still use the MASK feature with TTL transmitters but, you lose the ability to precisely control the number of shots, for each press and hold of the shutter button, in "continuous H" Drive. You also lose the ability to bracket ambient exposures, which is really nice when trying to control different ambient exposures across multiple images, when working outside with bright backgrounds. If you want to fire off a bunch of photos, with no set number of shots and are using the MASK feature to alternately fire your flash units to speed up the shooting speed and not be hampered by recycle times, then the following method is not needed and a TTL transmitter can be used.
Setting up the camera:
Go into the camera menu and scroll over to the first brown menu page "C.fn1:Exposure". Scroll down to "Number of bracketed shots" and choose 2 or 3. 2 shots works for most masking and flash/ambient exposure and flash contrast control setups. You can use 3 shots if you need an over exposed, ambient image for masking purposes although, 2 shots will more than suffice in most situations. Now scroll up one line to "Bracketing sequence". This setting is more important when using the MASK feature for exposure and contrast control because you will be using it to change the ambient exposure across multiple images. Use a setting that will place the subject flash exposure last. If you do use a 3 shot setup, then the best choice is to use "+0-". This puts your longest ambient exposure first followed by the subject flash exposure and the shortest exposure last. There is no setting that place's the subject flash exposure, represented as "0", last in the bracketing sequence. Use "-0+" for 2 shot setups when underexposing the background and "+0-" when masking in the studio. The assumption is that hopefully the subject will hold still until they see the flash exposure on them.
The next step is to scroll back in the menu to the second red page "shoot2". The first item is "Expo.comp./AEB" where you will set the bracketing, for your ambient exposure(s), when using the Mask feature to control the exposure and contrast outside. Set this to what ever you want the background underexposed in relation to the subject exposure. Usually "-1" or "-2" stops when using 2 shots and "-/+1" or "-/+2" when using 3 shots. In the studio set this to "+1/3" because even though there will be no need to effect the ambient exposure, you still need a setting to allow for precise control of the number of shots when you press and momentarily hold down the shutter button. Using "+1/3" also adds a little protection that if you use max sync for your manual shutter speed, the bracketed shot will be 1/3 stop less.
Lastly go to the top of the camera and set the "AF-DRIVE" to "one shot" for the auto focus and "continuous H" for the DRIVE. Be sure to put your camera on a tripod and set the camera to Manual exposure mode. In Manual exposure mode the camera will bracket the ambient exposure by changing the shutter speed and this also allows you to lock in the main/mid exposure for your bracketing. So now the camera is setup so that when you press and momentarily hold down the shutter button, the camera will fire the precise number of shots, set in menu setting "Number of bracketed shots".
Setting up the flash:
The MASK feature only effects that one individual flash and basically all it does is count how many time's the flash receiver sees a trigger signal from the transmitter. The flash fires when it's turn comes up, based in it's position in the shot sequence. So if you are taking 2 shots for masking, one of the background and one of the subject, the background flash units would fire in the first shot (position 1) and the subject flash units would fire in the second shot (position 2). The total number of shots is set in the flash menu under "UNITS" and the position, a flash will fire in the sequence of shots, is set under "ALT". In a nutshell the number set in the "UNITS" will be the same as the number set in "Number of bracketed shots" but not always, so you'll have to wait for my video titled "101 ways to use the Godox MASK feature" to see more advanced uses. 
Using the MASK feature to combat the sun:
This method works well when you want to maximize flash power by not exceeding max sync. It also helps to eliminate the need for ND filters or at least reduce the total number of stops of ND needed. We'll use a two shot setup with the first shot an ambient only exposure, under exposed by 2 stops, using the settings "-0+" in "Bracketing sequence" and "-2" set in "Expo.comp./AEB". The second shot will be the flash/ambient exposure at max sync speed. This method allows you to use a higher ambient exposure, with our flash exposure, to help not only lower the contrast in the shadows, cast by the flash but, also to use a lower flash power setting while making the flash exposure look less flashy. So what we have done is capture an image with the background underexposed using a shutter speed faster than x sync speed in the first shot while creating an ambient/flash exposure, at max sync speed, in the second shot. When you combine the two shots in Photoshop, you can quickly create a layer mask, by painting in the subject or the background, depending on how they are stacked in layers.
