Here is a picture of a sample label containing a PDF symbol.  On this symbol, some PDF Codewords have been "crossed out" so that we can illustrate the way that the TruCheck system identifies and reports this type of label damage.

Notice that the left part of one of the rows is crossed out.  Also, an entire column is crossed out.  You will see in the report a "map" of the label, showing these code words as having been unreadable.  Due to the error correction capability of PDF417, the overall data content of the code is still retrievable.

Error Correction Usage:

PDF417 symbols contain built in "error detection and correction" capability.  A certain number of extra code words are printed in the symbol in order to allow errors to be tolerated when the symbol is read.  An "error" is any code word which can not be read at all, or which the reading algorithm thinks it can read, but actually is read wrong (decodes as a different code word.)  Code words that cannot be read are called "erasures". Code words that are mistakenly read with the wrong value are called "errors".  The number of erasures and errors which can be tolerated depends upon the number of extra codewords that are printed in the symbol.  This is called the error budget and is determined by the "Security Level" of the PDF 417 symbol.  

Each "erasure" uses up one error correction code word from the error budge.  Each "error" uses up two from the budget.  The total number of code words used up is therefore two times the number of errors, plus the number of erasures.  The unused error correction is any left over budget after errors and erasures are corrected.  The percentage of the original budget that is left over, is graded as the UEC parameter (unused error correction.)  The overall grade for the symbol is the lower of the UEC grade or the ANSI grade obtained from the start/stop characters.

Below is a copy of the PDF report that TruCheck prints: