The default value is "ALL" which spell checks every textarea, HTML text input and rich editor on the page.
FieldsToSpellCheck is very diverse in that you can enter the ID or ClientId for any object in the webpage - and it can be spellchecked. This can also target an HTM element by its ID. E.g. FieldsToSpellCheck = 'textBox1';
FieldsToSpellCheck can also be set to a list of fields to be spellchecked together E.g. FieldsToSpellCheck = 'textBox1,textBox2';
FieldsToSpellCheck can be set to target entire types of HTML form element.
Rich HTML editors are automatically detected - whatever the trpe