E Errors, Level 1
E11, E21 and E31
Are the errors which occur at the first stage of error correction, C1. The first number in an "E" error is always references the number of errors and the second number always references the decoder level, in this case, C1, or Level 1.
The error type E11 means one bad symbol was corrected in the C1 stage. E21
means two bad symbols were corrected in the C1 stage. E31 means that there were three or more bad symbols at the C1 stage. This block is uncorrectable at the C1 stage, and is passed to the C2 stage. Because of the de-interleaving of the data between the stages, those three (or more) bad symbols are now in separate blocks, and so can be corrected by the C2 stage. Because of the interleaving, one uncorrectable symbol at the C1 stage can be turned into as much as 28 bad symbols at the C2 stage. This is why E12 is typically much larger than E31.