Words can be assigned to a category. By deselecting a category the words are kept in the dictionary but will be treated as invalid. Words that are part of of a deselected category are highlighted with yellow background in the word search dialog.
The kind of such a category is left completely to the user respectively the author of the dictionary. For instance verbs and nouns can be identified, or a bi-lingual dictionary can be created where just one language is used at once, and, applied in the German dictionary, different reference books can be selected. However, categories should be taken carefully into consideration, since loading might be slowed down unnecessarily. The standard category "unknown" can be renamed to something else but cannot be disabled. Words of a category that has been disabled by user are still available in the dictionary, and shown appropriate in the word search dialogue, but will not be used by the computer and require acceptance if placed by user.
A dictionary can be loaded during a game and categories can be selected or deselected after game start as well. Information about loaded file and settings are synchronized in network games.
Tournament Word List (TWL) is the official word authority for tournament Scrabble in the USA, Canada and Thailand. It is based on the Official Scrabble Players Dictionary (OSPD) with modifications to make it more suitable for tournament play. Its British counterpart is Collins Scrabble Words, and the combination of the two word lists is known as SOWPODS. (Wikipedia)