Reported value is below 0 is a warning that indicates that XBRL is picking up a value that is positive or negative and the system expects the opposite due to the nature of the account. For instance, when for Reserves you are stating a negative number, you will run into this warning since the nature of the account indicates that the figure should be positive.
Please note that this is just a warning, not a blocking error. It doesn’t block you from submitting your file. It may very well be correctly stated within the report, it is however something that ESMA wants us to point out and its function is strictly informative.
The 'Reported value below 0' warning means that XBRL is giving you a heads up to recheck the data. In many cases, it can be beneficial to double-check and make sure that the input numbers are correct.
If present, the “Reported value is below 0” warnings will appear in the xHTML section as below.
If you would like to investigate what exact number the system is referring to, you can click on the “eye” icon on the right. The system will take you directly to the place where the related figure appears.