Lexical Item vs Grammatical Item

Halliday (2004: 3):

But teachers of English have customarily distinguished between content words, like snow and mountain, and function words, like it and on and of and the; and it is the notion of a content word that corresponds to our lexical item.

Halliday (1985: 61):

Grammatical items are those that function in closed systems in the language: in English, determiners, pronouns, most prepositions, conjunctions, some classes of adverb, and finite verbs.  (Determiners include the articles.)