GRI 102: General Disclosures, which includes 102-41 Collective Bargaining, sets out reporting requirements on contextual information about an organization and its sustainability reporting practices. This Standard can be used by an organization of any size, type, sector or geographic location.
This disclosure (102-41) asks for the percentage of employees covered by collective bargaining agreements. It does not ask for the percentage of employees belonging to trade unions.
Collective bargaining refers to all negotiations which take place between one or more employers or employers' organizations, on the one hand, and one or more workers' organizations (trade unions), on the other, for determining working conditions and terms of employment or for regulating relations between employers and workers. Therefore, a collective bargaining agreement represents a form of joint decision-making concerning the organization’s operations.
By definition, collective bargaining agreements are obligations (often legally binding) that the organization has undertaken. The organization is expected to understand the coverage of the agreement (the workers to whom it is obligated to apply the terms of the agreement).
Collective agreements can be made at various levels and for categories and groups of workers. Collective agreements can be at the level of the organization; at the industry level, in countries where that is the practice; or at both. Collective agreements can cover specific groups of workers; for example, those performing a specific activity or working at a specific location.
See full background here.