The left hand side of the mod_datamenu configuration is the default joomla on, this page depict the settings on the right side.
Each module need a layout to display it's content, the mod_datamenu has different types of layouts. The function changes with the suffix.
The module mod_datamenu is used to
Display a navigation menu
Layouts without a suffix, or ending on _menu ( VN:1631) display a navigation menu ( not to be confused with a joomla menu)
Layouts with _list display a list of selected items.
Extra SEO content
Layouts can be customized easily
The setting of this parameter influences the filtering of the sub-menus. smart and mini deselect combinations of categories that would result in zero items. yep that's confusing: the setting 'max' could result in a selection like United-States + Quebec, which will result in zero hits (well some might argue that Canada is a kind of US state but still).
Used in combintion with 'adding content'
The module always needs a com_datafeeds menu item to work with. The datafeeds menu defines the filters, navigation menu order and text strings. If the module is displayed on a datefeeds menu page it will automatic use that one. If you have several datafeed menu's the module will adapt the the one currently visible, so in general you will only need one navigation menu on your site. The module adapts even if the default ID is set.
If the module is used on a non datafeed menu, for example on an article page the module has no reference to a datafeeds menu and it will diplay: nothing. In this case you will need the default id, this value points to the correct datafeeds menu. You will find this ID on the right site in the joomla menu manager.
This parameter sets the number of sub-menu levels shown, see this forum article for details
Number of items to be shown in the _list and _item layouts.
And sets the number of values shown in the menu's. That is, the first submenu displays all possible values, the other limit to items, then if the first submenu has a selected value the second has no limit, the deeper ones items etc.
Used in some menu's to define the number of columns
Impacts the _list and _items
Impact the _list and _items, like the group parameter in the datafeeds menu configuration it will show only one item for each grouped value