Difference between Chat Up and Set Phrase

CHAT UP vs SET PHRASE: NOUN

A common expression whose words cannot be replaced by synonymous words without compromising the meaning.
A common expression whose wording is not subject to variation.
An expression whose meanings cannot be inferred from the meanings of the words that make it up

To talk to (someone) in a friendly, open, or casual manner, sometimes also in a charming or affected manner, usually to curry favor, and sometimes flirtatiously with the intention of establishing a romantic or sexual encounter or relationship with that person.
Talk or behave amorously, without serious intentions
Talk to someone with the aim of persuading him

Cliched, Wittified, Hit on, Banteringly, Dysphemia, Flirtatious, Corny, Flirtingly, Coquet, Coquette, Dally, Mash, Philander, Romance, Flirt

Syllabism, Content word, Tralation, Italicism, Epanadiplosis, Noun phrase, Verb phrase, Hispanism, Dorism, Cockneyism, Figure of speech, Phrasal idiom, Idiomatic expression, Idiom, Phrase

Joke, Cheesy, Cliched, Wittified, Hit on, Banteringly, Dysphemia, Flirtatious, Corny, Flirtingly, Coquette, Dally, Philander, Romance, Flirt

Expressive style, Extradictionary, Syllabism, Content word, Tralation, Italicism, Epanadiplosis, Noun phrase, Verb phrase, Hispanism, Dorism, Cockneyism, Figure of speech, Idiom, Phrase

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Sometimes a collocation is meant as a synonym of a word combination, sometimes it is a special type of a set phrase.
As will be shown later, this set phrase appears not only in the SSP list, but is broadly seen in many other contexts.
Rude am I in speech, and little blest With the set phrase of peace.
English dictionaries as an idiom, set phrase, collocation, no matter whatever it is?