Universal Containers (UC) has a legacy system that needs to integrate with Salesforce.

To create a digest of account action plans using the generative API feature, Universal Containers should use the REST API. The REST API is ideal for integrating Salesforce with external systems and enabling interaction with Salesforce data, including generative capabilities like creating summaries or digests. It supports modern web standards and is suitable for flexible, lightweight interactions between Salesforce and legacy systems.

Metadata API is used for retrieving and deploying metadata, not for data operations like generating summaries.

SOAP API is an older API used for integration but is less flexible compared to REST for this specific use case.

For more details, refer to Salesforce REST API documentation regarding using REST for data integration and generating content.