Groups are containers for LightWave items. Groups can contain any kind of item, including Bones, although Janus does not process bones making them contextually meaningless.
Groups contain user overrides, which is discussed below. But most importantly, groups always have a type. A group type is a preset of group overrides. As such group types are user-definable. Janus, moreover, provides some standard group types you may find useful. But besides user-defined types, Janus has two important inbuilt types called NOM and NULL.
The NOM type (nominal) does not apply any new settings. This type is useful when you want a command to dictate visibility, which is usually preferable. Remember that a user override can be applied at any time. The NOM type just gives a blank canvas to work with, so to speak.
The NULL type definition is also straightforward. It simply 'deactivates' a group from the render pass. To 'deactivate' means to replace items in the group with nulls, so that they are not loaded when the scene is rendered. By default, any group with mesh items that is not associated with the command being run is 'turned off'. This makes NULL types redundant when using with meshes. However, this behavior doesn't apply to lights, which are turned on by default (though this may be changed through the DEFLIGHTEXCLUDE subcommand). Therefore, the NULL type is useful when you want to explicitly deactivate lights from a pass.
The NOM and NULL names are reserved, and Janus will ignore any user-defined group type with the same names.
Naming Rules
Duplicate names of the same type are also not allowed in groups. Although Janus does not consistently check for duplicate names of the same type, referring to group associations is name-based. If you have two groups called NOM*.item (hoping to use different overrides for each one), Janus will only recognise the first one it checks for. So be aware of this limitation. Rename your groups something else. However, it is totally acceptable to have same names where different group types are concerned. You may have NULL*.item, MAT*.item, and NOM*.item all at the same time.
User Overrides
User overrides are also called manual overrides because the user explicitly sets them in a Janus dialog. These settings override all other pre-set parameters.
Much of the easy functionality in groups are from user-defined group types. Find how to create user-defined groups in the Creating Your Own Group Types section.
Propagation
Propagation is an ability of a group to traverse down an item's hierarchy and apply its settings to everything below. Suppose you had a piece of landscape and wanted to duplicate it for ten times to spread it around. Take your landscape and 'group' it by parenting it to a new null object, and assigning that null object into a Janus group. Then duplicate your landscape, making sure that that duplicates are still parented to the null. As long as they are under it Janus will apply the settings to the null and everything beneath.
This allows for easy modification of assignments. You don't have to go back to the Janus panel to reassign new objects. Just make sure you are organised. Remember that if you want Janus to keep track of item name changes you must turn on the option.