What are Uncle blocks?

Uncle blocks (or Ommer) are created when two or more miners create blocks at nearly the same time . Only one block can be mined and accepted as canonical on the blockchain. The others are uncle blocks, which are not included but still provide a reward to their miners for the work done.

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What is a candidate block?

Intermediate. In a few words, a candidate block is a block that a mining node (miner) is trying to mine in order to receive the block reward . So a candidate block may be described as a temporary block that will be either validated or discarded by the network.

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