That's a strange way to report progress but it's the number of bits in SPI sprites that should be correctly decrypted at the moment. The encryption works on 48 bits at a time, and I have to decrypt each one of them.
Luckily, having decrypted Raiden Fighters 2 makes things easier because the first few sprites in Raiden Fighters are identical, so I can compare them and see where pixels should go. However, the process isn't easy because the SPI sprite encryption is quite more complex than the tile encryption. Of the 34 1/2 bits still left to do, 16 will be particularly hard because the encryption scrambles them more aggressively. The others shouldn't be too hard to do, given enough time and patience. I have plenty of the latter, but never enough of the former.