All distribution lists that are not system managed have access to a subscription rules tab, this tab allows you to set up some conditions for subscription. If a contact/org meets these conditions, they are added to the distribution list when they are saved or if the "run subscription rules against all contact records" button is clicked.
If a subscribed contact/organisation no longer meets the conditions or becomes expired, they are unsubscribed from the distribution list. The exception to this is manually subscribed contacts/orgs. Any contacts or organisations can be manually subscribed regardless of the subscription rules, and they must also be manually unsubscribed because they ignore the conditions set in the subscription rules tab.
The available distribution list tabs depend on which member types you have selected for the distribution list. The contact tab is used for subscribing contacts only, and the organisation tab subscribes organisations.
When you have selected a tab you must firstly select the email address type(s) that contacts/organisations will subscribe with. They must have an email of this type available in distribution lists to be subscribed. If a contact/organisation has multiple emails, they will sign up using the default email address.
After the emails have been selected, then click "maintain subscription rules" to create your conditions for automatically subscribing contacts/organisations. You select either the contact or organisation module under the field heading, the field name, then the operator and value under their respective headings. When subscribing contacts, the organisation tab is used for subscribing contacts based on any of their organisation's details.
There is also an "Add Group Condition" checkbox in this grid, that when clicked, changes the selected field to a group condition. You can have multiple group conditions, which allows you to set up multiple sets of rules in a single distribution list. For example (Condition 1 AND condition 2) OR (Condition A)
If you change the subscription rules, the system will ask if you want to remove the existing members who no longer meet the new rules (excluding those who were manually subscribed). If Yes is selected they will be removed.