A selected, or subcategorized, phrase is obligatory, as contrasted with adjuncts, which are optional. For instance, the direct object of a transitive verb is obligatory and therefore a complement, whereas adverbial modifiers are generally optional, and therefore non-complements. However, the distinction is not always clear, particularly for oblique arguments. Neither is the distinction clear in languages in which complements can be freely omitted if they are understood from the context. Omission must be distinguished from pronominalization; pronouns may generally be considered to be complements. However, in some languages pronouns have been grammaticalized as verbal affixes, in which case the question of whether they are complements or not becomes a theory-internal question. (See Baker 1996, chapter one for discussion of this issue.)

The complement/ adjunct distinction cross-cuts the core/ oblique distinction, since there are obliques which are complements, and other obliques which are adjuncts. Also, while the subject of a clause is often considered a core argument of the verb, it is not normally considered to be a complement. This is because in most (perhaps all) languages, the subject appears to be a clause-level constituent, rather than a constituent of the verb phrase. However, it should be noted that this argument presupposes that the verb and its object belong to the same phrase-level constituent, while the subject is outside that constituent, an analysis which leaves the status of subject and object in VSO languages unclear.