Conversation threading is a feature used by many email clients, bulletin boards, newsgroups, and Internet forums in which the software aids the user by visually grouping messages with their replies. These groups are called a conversation, topic thread, or simply a thread. A discussion forum, e-mail client or news client is said to have a "conversation view", "threaded topics" or a "threaded mode" if messages can be grouped in this manner.

Threads can be displayed in a variety of different ways. Early messaging systems (and most modern email clients) will automatically include original message text in a reply, making each individual email into its own copy of the entire thread. Software may also arrange threads of messages within lists, such as an email inbox. These arrangements can be hierarchical, arranging messages close to their replies in a tree, or they can be linear, displaying all messages in chronological order regardless of reply relationships.

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Tracker comments definitely are not threaded when everybody has to enter a comment topic.
Experiments on gmail show gmail groups emails from same sender by similar topics. For threading, the topics need to be exactly the same except some short string in front, like forwards, replies…
Gmail ignores all email standards threading cues such as fields In-Reply-To:, References:, Message-Id:. See

Specs / algorithm