This Spell check functionality checks any spelling mistake in the content which we are entering into the SharePoint List/Libraries. Suppose if we want to add some new words which should not be considered as spelling mistakes. We can do this by adding our custom dictionary. Let me show this using a example:
1. I will use Task List and in description I will put SharePoint in different ways. The only acceptable way is SharePoint. So my other Sharepoint and SharepoinT will be shown as errors.
2. Now I will create a custom dictionary. To do this create a new Document Library, "Spelling" at the root of the site.
3. Create a text file, "Custom Dictionary.txt" and add all the words that should not be recognized as spelling errors in the text file, one word per line.
4. Upload the text file to the newly created Library.