The biggest differences between the three game modes is their combat systems (and the accompanying magic/prayer systems). Although the packages of the following classes already reflect there usage, this should provide some further clarification. For OSRS, you need to use the Magic and Prayer classes to activate spells and prayers. For Legacy and EOC, you need to use the Powers class and it's respective sub-classes. For OSRS, you need to use the OSRSTab class to switch between tabs. For Legacy, you need to use the LegacyTab class to switch between tabs. For EOC, you need to use the ActionWindow class to switch between action windows. All three of these classes are unified for easier usage within the InterfaceWindows class (however only common functions are exposed there).