The basic element in Writer is a paragraph. A paragraph will receive text data and control data, such as breaks and spacing. In the end, your document is a sequence of paragraphs.
A blank document is reduced to an undeletable empty paragraph because this is the minimum.
You create new (empty when typing at the end) paragraphs with
Enter. The latter one adds to the paragraph object control information to cause a page break.
Formatting Marks enabled, when you see a single paragraph marker on a line, this paragraph really does not contain any text data. However there is always some form of control data with it, be it the space above and below this paragraph. Due to this control data, an empty paragraph is never “neutral”: it always occupies some rectangular space. This is this space Scribus takes into account (paragraph has no text but it does have a non-empty bounding box)
Using empty paragraphs to create empty pages is generally a bad idea because you are vulnerable to any change in formatting, like changing page style properties (margins, header, footer, …) which will mess up your manual layout. It is preferable to use page breaks because these are guaranteed to always be synchronised with pages.
If your need is more advanced, e.g. to always start a chapter on a right page, this can’t reliably be done with manual page break because you don’t known on which page (left or right) your text is presently laid out and this can change if you edit the document before this position. The way to handle this is with properties of page styles. These styles can be constrained to left or right pages and you tell writer to which style automatically switch when page overflows.
This is how built-in Left Page and Right Page work.
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