In software testing there are 4 Test Levels. They can be encountered when testing both a system and a system of systems (multiple, dispersed, independent systems in context as part of a larger and more complex system). If this article I am going to show my test manager view on the software testing test levels.
Component / Unit Test
Focuses on components that are separately testable.
Focuses on interactions between components or systems.
Focuses on the behavior and capabilities of a whole system or product, often considering the end-to-end tasks the system can perform and the non-functional behaviors.
Focuses on the behavior and capabilities of a whole system or product: confidence, completion, fit for use & purpose. Some call it user acceptance testing.
The best way to navigate across the software Test Levels is to follow a predefined course, to constantly monitor it as you go along and apply course corrections whenever you see fit.
This article is based on the ISTQB Advanced Syllabus version 2012 and it also references the ISTQB Foundation Syllabus version 2018. It uses terminology definitions from the ISTQB Glossary version 3.2.