If it is, then it must be included in the main text.
If, however, it is too lengthy or too detailed it might be better to summarise it, including the essential points in the main text, and then writing an appendix to place the complete material in its own dedicated section.
Again, it might be best for a reader to have all the essential information in the main text, instead of having to refer to an appendix, which can often be inconvenient and impractical.
However, if this means that the main text will be difficult to read because lengthy and detailed material will interfere with the general flow of the argument, then the writer should write an appendix and relegate material to this appendix.
Various academic and other institutions, industries and professions have their own preferred rules and conventions for structuring and formatting written texts and these should always be consulted when preparing to write an appendix.
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The purpose behind writing an appendix is not to create a place for information that cannot be conveniently accommodated in the main text.
To write an appendix it is important to understand the two major perspectives that must be served in any successful piece of writing.
This can apply to anything from lengthy quotations and long lists to detailed procedures and excessive raw data.
The second question to be answered when preparing to write an appendix is this one: Is it more helpful for the reader that this information be included in the main text or placed in a separate section?