This answer explains that an activity box marked "C Customer" means that the customer should be consulted.
In a process map that uses Universal Process Notation (UPN), an activity box is a symbol that represents a task or an action that is performed in a business process. An activity box can be marked with a letter to indicate the role or the resource that is responsible for performing the task or action. The letter C stands for consult, which means that the resource should be asked for input or advice before or during the task or action.
References: https://trailhead.salesforce.com/en/content/learn/modules/salesforce-business-analyst-quick-look
/use-process-mapping-to-understand-your-business