What does insuring agreement a ( d & o ) mean?

An insuring agreement is the main section of an insurance policy document. The insurer specifically mentions the risks that have covered in the contract under the insuring agreement. This section forms the basis to determine if the loss is covered under the insurance policy and both the insurer and the client will refer to this section in the event of a claim. 
Directors & Officers liability insurance typically comprises three insuring agreements: Side A, Side B and Side C.

• Side A  – Responds when a company is unable to indemnify its directors and officers. The most common example of such a situation is when a company becomes insolvent. Side A coverage is often referred to as the “personal protection” part of a D&O Insurance contract.
• Side B – is the part of a D&O policy that reimburses a company for its indemnification obligation to its directors and officers. Side B provides for reimbursement for the company.
• Side C is also known as “Entity Coverage”—is the part of a D&O policy that responds to securities claims made against the company.