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Wiktionary
line function

n. (context: management) Any primary business function that would affect customers or profits if interrupted

WordNet
line function

n. activity contributing directly to the output of an organization [syn: line duty]

Wikipedia
Line function

In management, a line function is any kind of daily operation such as purchasing, manufacturing, and selling that is directly involved in carrying out the purpose of an organization. This contrasts to a staff function, which involves the provision of advice and other support to those involved in a line function.