Report Substitution Fonts is a concept for runtime font substitution when formatting reports. Even if a report layout have been designed using a specific font, there's a possibility to have the framework use a different font when formatting the report. A typcial scenario when this functionality is useful would be when the report layout have been designed using a font that not capable of displaying all the glyphs used in the current translation/language your formatting for. The report substitution font functionality will enable an administrator to define a substitution fort to be used in runtime, replacing the design time defined font that's part of the report layout. This way a report layout designed using a font that only containing glyphs for western European characters can still be used in for instance Chinese. Simply define a Chinese substitution font for the font used in the layout. This eliminates the need to have different layouts, depending on language. It also eliminates the need to try and find a font that both contain all glyphs for all languages you need to support and at the same time look good in all languages (something that can be quite hard).
This functionality also enables you to adjust (increase or decrease) the font size - this in order to be able to compensate for undesirable layout effects caused by the fact that different fonts (of the same point size) take up different space.