An interested party is any person or organisation that can affect, be affected by, or perceive itself to be affected by a decision or activity. Clause 4.2 requires you to determine which are relevant to your management system, and what their relevant requirements are.
Who they usually are
Customers, employees, owners and shareholders, suppliers and partners, regulators, certification bodies, neighbours and local communities, insurers, and increasingly the customers of your customers where supply chain requirements flow down.
The word “relevant” is doing real work
You are not required to catalogue everyone with an opinion. Only parties whose requirements are relevant to the management system, and only those requirements. A supplier’s preference for longer payment terms is a real requirement of a real interested party — and almost certainly not relevant to your quality management system.
Requirements, not wishes
What you are capturing is what these parties require, particularly where those requirements become obligations. A customer contract specifying certification, a regulator’s reporting duty, a parent company’s group policy — these have consequences if unmet.
Where it connects
Interested party requirements feed the scope of your system, your risks and opportunities, and your objectives. In ISO 14001 and 45001 they connect directly to the compliance obligations clause, since legal requirements arrive through this route.
The practical version
A short table — party, relevant requirements, how we know, how we meet it — reviewed when something changes, is enough. It is one of the requirements most often over-engineered into a document nobody reads and least often connected to anything that matters.