The function/content distinction
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(mostly applied to words, perhaps because they are the most pliable among the (near-)surface forms)
Function words have little lexical meaning, and are instead used for grammatical assistance, possibly with a pragmatic aspect. They tend to be closed classes, often prepositions, auxiliary verbs, conjunctions, pronouns, articles, particles, expletives, and pro-sentences
Function words are a mostly closed class. Content words are an open class probably better seen as part of morphology - as content morphemes, as in derivational morphemes and inflectional morphemes (see derivational morphology and inflectional morphology).