Mod.cpp is file in your addon root directory, not <MyAddon>\Addons\ but in the <MyAddon>\ root dir. This file sets up how your mod is shown in-game ArmA 3.

Example mod.cpp:

/// Name of your mod
name = "PMC";
/// Picture displayed from the expansions menu/ Optimal size is 2048x1024, other sizes work too
picture = "picture.paa";
/// Display next to the item added by the mod
logoSmall = "logo_small.paa";
/// Logo displayed in the main menu
logo = "logo.paa";
/// When the mouse is over, in the main menu
logoOver = "logoOver.paa";
/// Website URL, that can accessed from the expansions menu
action = "https://www.pmctactical.org";
// name of the action button
actionName = "PMC Home";
/// Tool tip displayed when the mouse is left over, in the main menu
tooltipOwned = "PMC Since 1984";
/// Color used for DLC stripes and backgrounds (RGBA)
dlcColor[] =
{
	0.23,
	0.39,
	0.30,
	1
};
/// Overview text, displayed from the extension menu
overview = "Visit <a href='https://www.pmctactical.org'>PMC Tactical</a> official homepage.<br/><br/>Also <a href='https://pmc.editing.wiki'>PMC Editing Wiki</a>.";
/// Hide the extension name
hideName = 0;
/// Hide the extension menu
hidePicture = 0;
// author?
author = "Snake Man, PMC.";

After unpacking your addon the first thing user sees is contents of in-game menu and if you have bad or even missing mod.cpp it gives very unprofessional look to your addon.

Display name length is 30 characters for expansions menu. It can be longer, but the text is cutoff at least in this menu. If its longer it doesn't seem to cause any harm.

Overview string length is about 360 characters what you see below, however note that the html stuff does not count as character in arma3, you must subtract those or in case of <br/> it turns into line feed obviously.

You could also use class CfgMods to setup your mod in-game look or “mod presentation” as bis calls it, however that requires the user to keep your original addon directory name, if not then cfgmods breaks. Easiest solution is to use mod.cpp as it will always work.

The images defined in the mod.cpp can be stored in the mod directory or in the addon pbo file.

Recommended sizes

• Logo: 128×128
• LogoOver: 128×128
• LogoSmall: 64×64
• Picture: 2048×1024