In RESTful Application Programming, the EML statement that retrieves an object is GET entity. The GET entity statement is used to read data of an entity instance from the database or the transaction buffer. The GET entity statement can specify the entity name, the entity key, and the entity elements to be retrieved. The GET entity statement can also use the IN LOCAL MODE addition to bypass the access control, authorization control, and feature control checks. The GET entity statement returns a single entity instance or raises an exception if no instance is found or multiple instances match the key.
The other EML statements are not used to retrieve an object, but have different purposes and effects. These statements are:
* FIND entity: This statement is used to search for entity instances that match a given condition. The FIND entity statement can specify the entity name, the entity elements to be returned, and the condition to be applied. The FIND entity statement can also use the IN LOCAL MODE addition to bypass the access control, authorization control, and feature control checks. The FIND entity statement returns a table of entity instances or an empty table if no instances match the condition.
* SELECT entity: This statement is used to query data of entity instances from the database or the transaction buffer. The SELECT entity statement can specify the entity name, the entity elements to be returned, and the filter, order, and aggregation options to be applied. The SELECT entity statement can also use the IN LOCAL MODE addition to bypass the access control, authorization control, and feature control checks. The SELECT entity statement returns a table of entity instances or an empty table if no instances match the query.
* READ entity: This statement is not a valid EML statement, but an ABAP statement. The READ statement is used to access a single row of an internal table using the table index or the table key. The READ statement can also use the TRANSPORTING addition to specify which fields should be returned, and the INTO addition to specify the target variable. The READ statement returns a single row of the internal table or raises an exception if no row is found or multiple rows match the key.
References: GET ENTITY - ABAP Keyword Documentation, FIND ENTITY, ENTITIES - ABAP Keyword Documentation, SELECT ENTITY, ENTITIES - ABAP Keyword Documentation, READ - ABAP Keyword Documentation

On-stack developer extensibility is a type of extensibility that allows you to create development projects directly on the SAP S/4HANA Cloud technology stack. It gives you the opportunity to develop cloud-ready and upgrade-stable custom ABAP applications and services inside the SAP S/4HANA Cloud, public edition system. You can use the ABAP Development Tools in Eclipse to create and deploy your on-stack extensions.
On-stack developer extensibility is suitable for the following kinds of applications:
* Applications that provide APIs for side by side SAP BTP apps. On-stack developer extensibility allows you to create OData services or RESTful APIs based on CDS view entities or projection views. These services or APIs can expose SAP S/4HANA data and logic to other applications that run on the SAP Business Technology Platform (SAP BTP) or other platforms. This way, you can create a loosely coupled integration between your SAP S/4HANA system and your side by side SAP BTP apps.
* Applications that access SAP S/4HANA data using complex SQL. On-stack developer extensibility allows you to use ABAP SQL to access SAP S/4HANA data using complex queries, such as joins, aggregations, filters, parameters, and code pushdown techniques. You can also use ABAP SQL to perform data manipulation operations, such as insert, update, delete, and upsert. This way, you can create applications that require advanced data processing and analysis on SAP S/4HANA data.
The other kinds of applications are not suitable for on-stack developer extensibility, as they have different requirements and challenges. These kinds of applications are:
* Applications that integrate data from several different systems. On-stack developer extensibility is not meant for creating applications that integrate data from multiple sources, such as other SAP systems, third-party systems, or cloud services. This is because on-stack developer extensibility does not support remote access or data replication, and it may cause performance or security issues. For this kind of applications, you should use side by side extensibility, which allows you to create applications that run on the SAP BTP and communicate with the SAP S/4HANA system via public APIs or events.
* Applications that run separate from SAP S/4HANA. On-stack developer extensibility is not meant for creating applications that run independently from the SAP S/4HANA system, such as standalone apps, microservices, or web apps. This is because on-stack developer extensibility requires a tight coupling with the SAP S/4HANA system, and it may limit the scalability, flexibility, and portability of the applications. For this kind of applications, you should use side by side extensibility, which allows you to create applications that run on the SAP BTP and leverage the cloud-native features and services of the platform.
References: Developer Extensibility in SAP S/4HANA Cloud ABAP Environment, SAP S/4HANA Extensibility - Simplified Guide for Beginners