A segment is a saved filter. Rather than listing people, it stores the rule that describes them, and works out who matches every time you look.
That is the whole difference from a list: nobody is ever added to a segment. A contact is in it whenever they match, and drops out when they stop matching.
Why that is useful
Take a segment for everyone who arrived through an agent and has a phone number. Every new contact who fits joins it without you doing anything, and the count on your Segments page is accurate whenever you read it.
Maintaining the same group as a list would mean revisiting it every week.
What a segment can be built from
A segment saves whatever you had set on the Contacts page:
Your search text
Source
Company
Whether the contact has an email or a phone
A list they belong to
Added after a date
A segment can include a list as one of its conditions, which is a neat way to say everyone in this list who also has a phone number.
Using a sement as a broadcast audience
Segments are made to be sent to. When you compose a WhatsApp broadcast you can pick a segment as its audience, and the send goes to whoever matches at that moment.
Two things to keep in mind. Only members with a phone number can receive a WhatsApp message, and the composer shows you how many of the segment are reachable. And because a segment is live, its size can change between writing the broadcast and sending it.
Check a segment before you send to it
Saving a segment with no filters set is allowed, and it produces a segment that matches every contact you have. Mando warns you at the time, but it does not stop you.
So before using a segment as a broadcast audience, open it and look at the count and the members. It takes a moment and it is the difference between reaching a group and reaching everyone.
Frequently asked questions
Can I edit a segment rule later?
You can rename a segment, but not change its rules. Build a new one from the Contacts page instead.
Does deleting a segment delete contacts?
No. The confirmation says so, and your contacts are untouched.
How many segments can I have?
Up to 200.