They have been described as those sources closest to the origin of the information or idea under study.
[1][3] These types of sources have been said to provide researchers with "direct, unmediated information about the object of study.
For example, a history book drawing upon diary and newspaper records.
These are often meant to present known information in a convenient form with no claim to originality.
The letters after an item describes generally the type it is (though this can vary pending the exact source).
A source that is official is called authoritative if it is known to be reliable and its authority or authenticity is widely recognized by experts in the field.
Libraries specialize in collecting these types of resources so that students and faculty have the tools they need to research effectively.