An atomic service is defined as undividable from the perspective of the Execution Environment. For example if you deploy a Wiki software on the IaaS level, it is divided in the single services you have to engage/deploy to fulfill this, e.g. the operating system, the apache web server, the database, etc. Therefore you can change components of this Wiki software, so it is not an atomic service. Whereas if you rent this service from a SaaS provider and the Execution Environment does not deal with its component, it is considered as atomic service.