You can define and use as many custom user dictionaries as you want. In addition to the standard function of storing words not contained in your main dictionaries these dictionaries also have two more very useful functions:
The Auto-Correct function automatically replaces common typing errors and abbreviations with the correct words or entire phrases during manual spell checking (not during live spell-checking or typing). See Spell checking in the Creating and Editing Topics chapter for details on using this function. This function is not available in live spell check mode!
The Excluded Words function allows you to enter words that will always be marked as incorrect, even if they would normally be considered correct. This allows you to enter exceptions for words contained in the standard main dictionaries that you want to handle differently.