Thunderbird/Virtual Message Categories
Virtual Message Categories, or VMC's, are an advanced form of Saved Search Folders.
A message is simply an e-mail, although more types may be added in the future (SMS, IM). A message can belong to zero or more categories.
A category is in many ways similar to a folder. It is a named collection of zero or more messages.
A category can have zero or more associated criteria - messages that satisfy all of a category's criteria belongs in that category.
A category can have zero or more subcategories, which are categories whose included messages must match not only the criteria of the category itself, but also those of its parent category, supercategory.
The root category is the top category - all other categories are descendants of it. The root category includes all messages.
A message can also be added to a category explicitly, by user action or an automated process.