But what kind of data is stored there? Vins, vouts or both?
A simple web search reveals your answer.
The
rev*.dat files (the "undo files"), contain all UTXOs spent by the inputs of a block. It was introduced in Bitcoin Core 0.8, and contains a concatenation of records, one for each block. This mimicks the structure of the
blk*.dat files which contain blocks.
Each block record consists of:
- 4 bytes: network magic (0xf9,0xbe,0xb4,0xd9)
- 4 bytes: size of the CBlockUndo record (LE32)
- data_size bytes: CBlockUndo record
- 32 bytes: double-SHA256 of the serialized CBlockUndo record
[...]