When writing, it is important to recognize the support from the documentary sources you used. Otherwise, you commit plagiarism by passing off the work or thoughts of another as your own. It is also essential that readers be able to find and consult these sources and use them in their own work.
For example, if a technical report or maintenance procedure simply states that "we used a Canadian vibration standard", how the engineer is supposed to find the appropriate standard without its number or publication year?
In written works: you must cite ALL your sources, even if you are paraphrasing an idea, method, theory, statistic or piece of information, whether from:
For example: "Heat pumps account for x% of the air conditioning market in Canada" or "The hike in crude prices is due to (X, Y or Z)," are statements that must be linked to a source if the idea or the data does not come from you!
There is only one exception: if the information is common knowledge (Paris is the capital of France), citation is not required.