In the context of ISO 27001, ‘business-relevant data’ are those identified as:
paramount for the achievement of business objectives, results, and outcomes;
impacted by the most relevant risks identified in the risk assessment;
related to the fulfillment of legal requirements (e.g., laws, regulations, and contracts).
In short, they are the information that will cause the most negative impact in case their confidentiality, integrity, and/or availability being compromised.
In the Risk Assessment Table template included in your toolkit, you have a tab with examples of information security assets, and there is a specific category about data and information. This template is located in folder 5 Risk Assessment and Risk Treatment.
These materials will also help you regarding ISO 27001 and information identification: