In Vlocity Context Rules, when creating a source expression in a context mapping, the correct format to reference an sObject field is %%field%%. This format is used within the Vlocity platform to dynamically insert the value of a field from the context into the rule or mapping. This syntax allows the context rule engine to correctly interpret and retrieve the necessary data from the Salesforce object model, ensuring accurate and relevant rule processing based on the current transactional context.