In short
A minor error on the UK practical test. You can have up to 15 and still pass.
A driving fault (sometimes called a 'minor') is a small mistake that did not cause an actual problem — for example a slightly late signal at a turn, or a hesitation at a clear junction.
You can accumulate up to 15 driving faults and still pass. The same fault repeated four times is usually escalated to a serious fault, because it indicates a habit rather than a one-off.
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