The auto-increment options for Manual properties (integer or character) increment drawing-wide, not on a per space basis. If you want the first door opening in every room to be A, the second (if there is one) to be B, etc., then use a text-type Manual property for the suffix, set the default value to be "A" and then manually enter the "B", "C", etc. when there are multiple door openings in a room.
You do not need to use the Drawing Management feature (Project Browser/Project Navigator) to have door tags that show the room number and a suffix. You will have to make some edits in the out-of-the-box content. You will need a Location property that reads in the property from your Space that holds the room number. (The out-of-the-box RoomNumber property is going to read the project-based room number, which concatenates the Level of the Construct in which a Space resides with the auto-increment "Increment" property. This will have no value outside of the Project Navigator environment.) You could then create a tag that displays the room number and your door suffix in two separate view block attributes, or that displays the value of a formula property that concatenates the two.
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David Koch
Autodesk Discussion Group Facilitator
Using ADT 2004 at work; access to 2005, 2006, 2007 & 2008 at home