Relevance is the safety net that keeps Search Terms sensible - it's meaning, not just matching words.
Every term gets a 0-100% relevance score for how well it fits the keywords in its ad group. High relevance supports adding a term; low relevance supports blocking or reviewing it - unless performance proves otherwise. It's what stops a one-off fluke being added, or a clearly-relevant term being blocked before it's had a fair chance.
Keywords you add default to exact match (short, safe terms may use phrase).
Negatives are exact by default too, so a negative can never accidentally carry a core money-word and gut a campaign. Phrase is available when you choose it; broad is never used for auto-applied negatives.
There are really two "match types" in play, and Search Terms keeps them separate: how a search matched your keyword, and how a new negative will be added. They're never confused.
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