Note: This functionality is only available to users who have the D365 Marketing add-on/application.
Subscription Lists and Marketing Lists are very similar. That’s because Microsoft built the Subscription List entity/table on top of the Marketing List entity. So it’s really just a sub-type of a Marketing List, technically speaking.
1. Data Setup Differences
Looking at the records, the only data difference you can see between what is a Subscription List or Marketing List is the "Subscription" field. If set to true, it is a Subscription List.
2. Functionality Differences
Functionality-wise, Subscription Lists can give your portal users a way to opt-in or out of the list. You can also use Subscription Lists in your Customer Journeys and Marketing Forms.
You can review more details about how to incorporate Subscription Lists into forms here.
In contrast, Marketing Lists can't be opted out of by the user with out of the box functionality in D365 Marketing. In this case, if someone reaches out to you and requests to be removed from the Marketing List, you can manually accommodate the request in CRM. If there is a contact update form in the portal where users can opt out of bulk emails, for example, that would prevent them from receiving that type of communication entirely. They wouldn't get any emails from the system.
To further this point, you might have a Subscription List for a monthly newsletter. Users can opt out of receiving the newsletter subscription in the subscription center, but they would still receive other types of communications, like emails from the system.
Also, please note that Marketing Lists can't be used Marketing Forms or in Journeys/Customer Journeys. Again, Subscription Lists can be used in these. Segments can be used as well, so if you absolutely want to use a Marketing List, see our article on using a Marketing List in a Segment.