The "mainmenu" setting:
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The first term is text that appears on the menu.
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The second term is a hyperlink to take when a user clicks on the entry. Hyperlinks that start with "/" are relative to the repository root.
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The third term is an argument to the TH1 "capexpr" command. If capexpr evalutes to true, then the entry is shown. If not, the entry is omitted. "*" is always true. "{}" is never true.
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The fourth term is a list of extra class names to apply to the new menu entry. Some skins will classes "desktoponly" and "wideonly" to only show the entries when the web browser screen is wide or very wide, respectively.
Some custom skins might not use this property. Whether the property is used or not a choice made by the skin designer. Some skins may add extra choices (such as the hamburger button) to the menu.