It enables Magento and third-party developers to report system dependencies through composer.json files and, consequently, guarantee compatibility among Magento versions.A service contract includes data interfaces, which preserve data integrity, and service interfaces, which hide business logic details from service requestors such as controllers, web services, and other modules.- Magento is a modular system that enables third-party developers to customize and overwrite core parts of its framework. This flexibility comes for free.
A service contract is a set of PHP interfaces that are defined for a module
devdocs.magento.com/guides/v2.0/extension-dev-guide/service-contracts/service-contracts.html
