System integration testing and acceptance testing are two different levels of testing that have different objectives and stakeholders. System integration testing is the process of testing the interactions and interfaces between different components or systems that form a larger system. System integration testing verifies that the system meets its functional and non-functional requirements and works as expected in its intended environment. System integration testing is usually done by testers or developers who have access to the system architecture and design specifications. Acceptance testing is the process of testing the system by the end users or customers to determine if it satisfies their needs and expectations and is ready for deployment or delivery. Acceptance testing verifies that the system complies with the user requirements and business processes and provides value to the stakeholders. Acceptance testing is usually done by users or customers who have access to the user requirements and business scenarios. You can find more information about system integration testing and acceptance testing in A Study Guide to the ISTQB® Foundation Level 2018 Syllabus, Chapter 2, Sections 2.2 and 2.31.