People keywords are stored in separate table from standard keywords (i.e. AgLibraryKeywordFace). This allows them to be filtered separately from standard keywords (see screenshot). Only People keywords can be assigned to a face in People view and any name assigned to a face will automatically be listed as a People keyword (see red boxes in screenshot). Also, when exporting a photo you can remove the face/keyword data applied in People view from the file (Remove Person Info) whilst leaving all other metadata, keywords, etc.
Only People keywords can be assigned to a face in People view
Minor correction: You can assign both person and non-person keywords in People view. However, LR only suggests person keywords for faces, and if you assign a non-person keyword, it will be automatically converted to a person keyword.