This bar shows you what proportion of the transport unit you have used. You can use the loading volume used when calculating the maximum permitted quantity per transport unit.
You have four empty IBCs that have previously contained an organic peroxide with UN number 3105. All IBCs still contain some residue, on average 2% per IBC. Four IBCs together use a loading volume of approx. 7%.
In the table in section 22.214.171.124.3 of the ADR manual you will find that the maximum permitted quantity per transport unit for a substance with this UN number is 333 litres. The maximum permitted quantity for this shipment is therefore 333 x 0.07 = 23 litres.
The four IBCs together still contain 4 x 1000 x 0.02 = 80 litres of residue.
The permitted quantity per transport unit is exceeded; you should remove the existing quantity of residue before presenting the IBCs.