An important feature of the RadSpell is its ability to create custom dictionaries for every language. If you set the AllowAddCustom property to true an "Add to custom" button will appear in the RadSpell dialog. If an unknown word is found the user can add it to the custom dictionary, which will automatically be created.
Custom dictionaries are text files located in the folder specified in the DictionaryPath property ("~/App_Data/RadSpell/" by default). Their names are composed of the language name + suffix, specified in the CustomDictionarySuffix property. The default value of the CustomDictionarySuffix property is "-custom", so with the default values, the custom dictionary name will be "en-US-custom.txt".