You should create a custom dictionary if you need to add terms to the core Search dictionary. You should create multiple dictionaries if your added terms need to be administered separately, or applied differently to different indexes.
For example, your company sells both construction materials and heavy equipment. You maintain two separate indexes for customer searches, and a third, combined index, for internal use. You would create two term dictionaries, one for material terms and one for equipment terms. Each customer-facing index would include the related dictionary, and the combined index would include both.
