Marketing → Segments holds saved conditions. A segment is not a list of people: it is evaluated again at every send.
Conditions
A segment has a name, a description and up to 50 rules; every rule must match. Each rule is a field, an operator and a value (up to 500 characters). Operators depend on the field type: text (equals, not equals, contains), number (equals, not equals, greater than, less than), date (after, before), tags (contains).
Fields differ by group. Customers: email, created date, province, city, company type, company name, email verified date, last login date, assigned to. Contacts: email, created date, mobile number, linked customer, position, lead score, tags. Leads: email, created date, province, city, company type, company name, assigned to, mobile, lead status, lead source. A rule on a field a group lacks leaves that group out.
The counts
"Matches N people" and the Contacts column use a separate, simpler count than a real send. They include people without an email address and people on the suppression list, so the real send is smaller. The column is cached for about 20 seconds.
Trash
Deleting a segment moves it to Trash. Campaigns that used it keep the link and will refuse to send until you restore the segment or pick another. A segment still referenced by any campaign cannot be deleted permanently.