Contacts can be set as Members or Nonmembers
You can see if a Contact is a Member or Nonmember at the top of their Contact Detail drawer. Active AMS-Synced Contacts come over as Members. Members are automatically added to the Mandatory Topic.
Manually uploaded contacts show as Nonmembers.
NOTE: Your Topics can be made available to Members or Nonmembers or both, whether a Contact is a Member or Nonmember may affect which Topics they can opt into.