SDS Studio™Definition: The index number is the identifier of an entry in CLP Annex VI, in the form 603-002-00-5, used to pinpoint the legally binding harmonised classification of a substance.
One CAS number can map to several Annex VI entries, for example with different purity or physical form. The index number tells you exactly which row applies.
When a harmonised entry exists, its index number belongs in SDS section 3.