Separating Keywords in MetaImage

What is the correct way to separate IPTC keywords please? Should keywords each be separated by a space, a comma or a semicolon for example? Or should each keyword be added as a separate tag, with multiple keyword tags added (if that’s allowed)? What about if the keyword is actually a term or phrase, e.g. ‘oil painting’ which is more meaningful than entering ‘oil’ and ‘painting’ as separate entities? Thanks.