symbian os belongs to the embedded operating system family
An embedded operating system is an operating system for embedded computer systems. These operating systems are designed to be compact, efficient, and reliable, forsaking many functions that non-embedded computer operating systems provide, and which may not be used by the specialized applications they run. They are frequently also real-time operating systems, and the term RTOS is often used as a synonym for embedded operating system.
An important difference between most embedded operating systems and desktop operating systems is that the application, including the operating system, is usually statically linked together into a single executable image. Unlike a desktop operating system, the embedded operating system does not load and execute applications[1]. This means that the system is only able to run a single application.
so i guess it comes under Embedded software development