A breve is twice as long as a semibreve. You wouldn't use a breve in, for example, common time, because a semibreve takes up all the beats in a measure. But in other time signatures you might see it.
The Breve is not common in instrumental music, I can't think of an example off hand in piano music. Also the Semibreve rest is used as a whole bar rest regardless of the time signature (except 4/2).
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