The (D) stands for direct. This is contact method done outside of the scheduled follow-up. A scheduled follow-up is any follow-up that populates under the My Profile section Follow-up. For example, a direct contact can be made through a prospect or member's "Contact Now" link as seen at the top banner when viewing their account.
A scheduled membership follow-up performed through your profile page will reflect as a (M) follow-up on your Dashboard for prospects you are trying to sell a membership to, and active members that you are trying to generate referrals from.
A scheduled PT follow-up performed through your profile page will reflect as a (PT) contact on your Dashboard for active members you are attempting to sell personal training to.