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“Author Base” value shouldn’t match any other slug in your site. That would cause redirection issues. So, you cannot type in something like “writers” if you already have or plan to have a page named “Writers”, which default slug is/would be “writers”.
It make senses to set an “Author Base” in the singular, rather than the plural.
Guest “Author Base” could match User “Author Base”. Both of them are considered as “Authors”, so it makes sense to keep the same URL structure for both.