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source material is used when you want to include someone else's idea in your writing. So as to avoid plagiarism, it is advisable to cite the source material using the format you have selected for your writing. Also include the source material in the references list.
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source material is where you get information about something from. It may be a text book, a journal, a website or even any writing by any author. Using this content requires you to reference the source so as to avoid plagiarism.