It is used to communicate potential hazards during transport, handling, storage and disposal. Crucially this does not account for “risk”. To use an analogy, crossing the road is intrinsically “hazardous” because there is the potential for harm, but if the road is clear then your risk of being harmed is low.
A classification can either be self-imposed by industry (self-classification) or officially set by a regulatory authority (harmonised classification). Under laws of certain jurisdictions such as the EU, industry may be required to ‘self-classify’ substances as new knowledge uncovers adverse properties of the chemical.
Cobalt is self classified by industry as a carcinogen “1b” via the inhalation route of exposure. Once the hazard category is determined, the information is communicated accordingly via a label and a (Material) Safety Data Sheet (MSDS or SDS), for the purposes of handling, transport, use and disposal. Specific internationally recognised symbols are associated with each hazard category.