This chapter describes the formal TLA + models of the OpenComRTOS Layer 1 Interaction Entities (L1-Entities). The L1-Entities represent the API (Application Programmer’s Interface) of OpenComRTOS used by the Task entities to build up the application. There are also so-called Layer 0 entities but these are not accessible to the user. The L1 Entities are also all derived from a common so-called Hub Entity. As we will see later, the L1-Entities represent services the operating system offers to the user. The L1-Entities names represent the type of service they provide.