What is expected of the companies? Which concepts the companies should be familiar with? What is the extent of the responsibility? The OECD Guidelines distinguish between direct and more indirect responsibility.
First and foremost, the Guidelines state that the individual company is expected to avoid causing or contributing to adverse impacts on matters covered by the Guidelines. If a company do cause or contribute to adverse impact, they are expected to mitigate the impact.
"For the purposes of this recommendation, ‘contributing to’ an adverse impact should be interpreted as a substantial contribution, meaning an activity that causes, facilitates or incentivises another entity to cause an adverse impact and does not include minor or trivial contributions." For more information, read the OECD Guidelines for Multinational Enterprises, paragraph 14 of the Commentary on General policies.
In other situations, the individual company may play a more indirect role. When a company has not caused or contributed to the adverse impact, but the impact is nevertheless directly linked to its operations, products or services, the company is expected to use its leverage, acting alone or in co-operation with other entities, to influence the entity causing the adverse impact, to prevent or mitigate that impact.
The OECD Guidelines recognise that ‟there are practical limitations to the ability of enterprises to influence the conduct of their business partners. The extent of these limitations depends on sectoral, enterprise and product characteristics such as the number of suppliers or other business partners, the structure and complexity of the supply chain and the market position of the enterprise vis-à-vis its suppliers or other business partners”. (The OECD Guidelines for Multinational Enterprises, paragraph 14 of the Commentary on General policies).
You can read more about this subject in the chapters General Policies and Human Rights in the OECD Guidelines.