You can find information on each message by searching the validation code in the Technical Output Specification. In the Find and Replace window, where it says within, select workbook, so that it searches all tabs.
Once you have located the code, you can look at the columns ‘Validation Rules’ and ‘Record Level Error/Warning Messages’.
The column Recvd Data Item Blank tells you whether the record will be rejected if you leave this field blank. An N/A in this column means you won’t receive a reject or a warning if you leave that field blank. If a field is mandatory, then Reject will appear in this column, which means that the record will be rejected if you leave the field blank.
The column Format Error is similar, but instead of checking whether the field is blank, it is checking whether what you have entered is in the right format. The right format is specified in the column Format.
The column National Code Error tests whether information entered is one of the options applicable to that field, either using the codes listed in the column National Code or the ODS record of organisation codes (found by searching the ODS Portal or a full list can be downloaded from ODS data downloads.
The column Additional Validation Rules describes any rules (in addition to the three columns just mentioned) that are applied to the data.
The column Record Level Error/Warning Messages explains exactly what each warning or rejection code means. This includes all errors covered by the previous four columns.