Design a BPMN 2.0 model for a customer complaint handling process in a retail store. The process starts when a customer submits a complaint. A customer service representative (CSR) receives the complaint and logs it into the system. The CSR then assesses the severity of the complaint based on predefined criteria. If the severity is high, the process is escalated to the manager for further action. If the severity is low or moderate, the CSR proceeds to investigate the complaint.

During the investigation, the CSR contacts the relevant department or personnel responsible for addressing the complaint. They gather information, assess the situation, and propose a resolution. If a resolution is agreed upon, the CSR updates the customer about the proposed solution and awaits their confirmation. If the customer accepts the proposed solution, the complaint is marked as resolved, and the process ends.

However, if the proposed solution is rejected by the customer or if the investigation reveals a need for further action, the process enters a loop. The CSR continues to work on the complaint, consults with necessary parties, and proposes alternate solutions until a resolution is reached and accepted by the customer.

Throughout the process, the system sends automated notifications to keep the customer informed about the progress of their complaint. The notifications include updates on the investigation, proposed resolutions, and final resolution status.

Additionally, if the complaint remains unresolved after a specified period, the system escalates the complaint to the customer service manager for review and intervention.

Ensure that the BPMN model captures the main steps, decision points, loops, escalations, and notifications involved in the customer complaint handling process."