The number after the error identifier indicates the order in which Polyspace performs the checks.
Depending on the mode and filter selected, some checks may be hidden from the procedural entities view. This may make it appear as though the identifier numbers are not following a sequence.
In order to see the complete sequence, you can select 'All' in the 'Review level' slider and unselect all filters to see all the checks that Polyspace performs. The identifier number follows the order in which Polyspace performs the checks. Please note that this numbering starts from zero for every function.
This information has been added to the Polyspace Documentation (R2013a).
<http://www.mathworks.com/help/releases/R2013a/polyspace_c/c_gsg/review-verification-results.html?searchHighlight=idp.9>
For example, with IDP.9:
IDP stands for Illegal Dereferenced Pointer
9 states that the check is the ninth check in the function
