Generally speaking, sources are considered to be original or derivative. The words mean what they say, but sometimes there can be confusion. The original is the first one–the actual letter your relative wrote (the physical piece of paper they touched and used their writing utensil on). Any picture, transcription, scan, photocopy, etc. is a derivative.
Some derivatives are the legal equivalent of the original–the record copy of a deed or a will that is recorded in a records office. Some derivatives are mechanical reproductions that reproduce the document faithfully (unaltered color photographs for instance).
Calling something original or derivative is simply referencing its creation. Whether that something is accurate is another story.