Document A and Document B can contain anything in them, but they can have a lot of similarities but also contain certain differences which makes them both distinct and unique.
As a result of this, we can see that some sources have many similarities in them, but they also have differences.
For example, we can see that an autobiography talking about the life of Mahatma Gandhi and a newspaper article which wrote about him would have some similarities about the life of Gandhi, but they would also have differences because they are obviously written from different points of view.
Please note that your question is incomplete dso I gave you a general overview so that you can better understand the concept.
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