Combi ovens allows users to heat with dry heat only, steam only, or a combination of steam and dry heat. The above table tries to correlate the different names used for various settings.
STEAM The steam setting is pretty simple - a boiler boils water until it turns into steam, and the steam is introduced into the oven. If you set the temperature for steam at 180 F, for example, it will add hot steam until that temperature is reached, then stop adding hot steam till it cools a little, then adds more hot steam to maintain that temperature. Note if the mode allows you to go above 100 C or 212 F, it is adding dry heat as well as steam.
CONVECTION For most combis, there is a heating element , the convection element, at the rear of the oven, though normally it is hidden, and a fan blows air oven that heating element , and circulates it around the oven cavity. Your regular oven usually has an element that is at the bottom of the oven, or below the oven floor, it heats up and that heat rises through the oven cavity, without use of a fan. For gas ovens, instead of an electric element, you will have a gas burner, but again, it likely will be below the oven floor. Your regular oven also usually has another element at the top of the oven cavity, called the broil element, When that heats up, it heats up the oven, and can brown food that is placed nearby. Many combis only have a convection element at the rear, though some like the Wolf , have a broil element. Such ovens may offer combinations of convection and broil elements, but that will not be addressed here.
COMBI MODE. In some combi modes, some of the heat is generated by the rear convection element, and some is introduced by the hot steam from the boiler. Different manufacturers have various names for the options. To me, Gaggenau is the easiest to understand, and I used that as the basis for all the descriptions at the top. At 0% humidity, all of the heat is supplied by a dry heating element, and a vent is opened so that any moisture that comes out of the food being cooked is vented out of the oven. This is similar to your regular non combi oven. At 30%, all of the heat is still supplied by a dry heating element, but the vent is closed, so any moisture in the oven stays in the oven. This is in some way similar to cooking in a dutch oven with the lid on. For the rest of the settings, some amount of the heat is also supplied by steam. Note that once you go above 100 C or 212 F, you will not see steam condensing on the interior of the oven window. Further, while Gaggenau offers settings up to 100%, that is similar to saying it is running the boiler at full speed, not that 100% of the heat is supplied by steam.
TABLE To create this table, I read the use and care or instruction manual for the model listed, and then determined which modes were the equivalent of the setting described above based on the descriptions in the manual . Some of the values were clear from the manual - for example, the Vzug manual says that Hot Air Humid does not add any steam, but closes the vent. Others involved guessing on my part - the Thermador has a slow cook setting , which has a temperature limit, but my guess is that the vent is sealed.
If you have any question about whether steam is introduced during a particular mode, you may be able to determine it from the sound the machine makes - on my Viking you can hear the boiler injecting steam, or you can try to run a mode with no water tank, for those that are not plumbed, and that should let you know pretty quickly whether the boiler is being used.