A single font label can map to several physical font files (regular, bold, …) by registering a font configuration before the designer or report loads.

Register a font configuration.
var myFontCollection = [
    { fontName: 'MyFont', fontStyle: 'Regular', fontPath: '../Fonts/Arial.ttf' },
    { fontName: 'MyFont', fontStyle: 'Bold',    fontPath: '../Fonts/Ariblk.ttf' },
];

Stimulsoft.Base.StiFontCollection.registerFontConfig(myFontCollection);

How it works. One logical font name resolves to the correct file per style, so bold and regular text render from separate .ttf files under a shared label.