This Counter64 counts the total bytes submitted to the engine for decompression on this slot, regardless of whether decompression succeeded, and is documented to always be equal to or smaller than cnt2DecompressedBytes. An administrator uses this as the before-decompression baseline against cnt2DecompressedBytes to gauge how much data expansion decompression is producing on that slot. For example, a large gap between cnt2BytesToDecompress and cnt2DecompressedBytes on this slot shows the engine is expanding a lot of previously compressed data.