This panel is laid out as a tree structure. Each leaf is internally associated to a query; when a tree item gets selected, the associated query is run, and a new page is shown with the resulting list of messages. Each leaf is a selector.
The numbers inside parentheses represent the estimated number of messages matched by the selector. Bold text indicates that some of them are unread. Within the branch of current messages, when there are two numbers separated by a < (left than) sign, the one at the left is a count of current messages, and the one at the right is a count of unread messages. For example, in the screenshot, the branch Current messages->Tagged->qt-interest has 39 current messages, including 28 unread.
The built-in branches are:
Unread messages->All: messages that have never been read.Current messagesgroups selectors for non-archived messages. These are messages that need to be acted upon in a near future. Current is a synonym for "non-archived" or "not yet processed".Current messages->Tagged: messages to which a tag has been assigned. Each tag for which there is at least one current message is displayed in this branch. When tags are part of a hierarchy, the hierarchy is displayed as flattened text in a single line, with a right arrow representing the parent->child relationship.Current messages->Prioritized: messages that are curent and have a priority field whose value is greater or equal than 1.Current messages->Not tagged: messages to which no tag has been assigned.Archived tagged mail: messages that have a least one tag assigned to them, and have been archived. There are no counters computed for these leafs for performance reason, since it is expected that the number of archived messages may be huge.Sent mail: messages that have their status "sent" flag set to "Yes".Trashcan: messages that have their status "trashed" flag set to "Yes".User queries: queries that have been saved in the Query Selection by the command or set up by an administrator. Every built-in selector mentioned above could also have been created as a query with the corresponding SQL statement.
Important
Contrary to folders in traditional mail programs, selectors aren't necessarily mutually exclusive. In particular, new messages are also current, and theAll selector will obviously include the results of the more specialized Prioritized, or Not tagged selectors.