The default custom dictionary is the dictionary to which words are added when you click Add to Dictionary when running a spell check on your document. There can only be one default dictionary per language.
To specify the default custom dictionary:
Select Tools > Options from the menu bar. The Options dialog box is displayed.
Select Editor > Spelling from the navigation tree. The Spelling settings are displayed on the right.
Click Custom Dictionaries to display the Custom Dictionaries dialog box.
Select the custom dictionary that is to be the default dictionary for that language, and click Change Default.
Click OK to save changes and close the Custom Dictionaries dialog box.
Click OK to save changes and close the Options dialog box.
The dictionary is now the default custom dictionary for the specified language.