Is software written for a particular platform?

Software may or may not be written for a particular platform. The Word "platform" has multiple meanings; one must know the context in which it is used to determine which meaning is intended.

The oldest meaning is "hardware platform"--a specific instruction set architecture. This is what was originally meant when referring to "platform-independent" code; portable software could be compiled for different processor architectures without modification.

In some contexts, the "platform" was expanded to include the operating system, as well. Interpreted languages like Java began to provide networking and user-interface libraries that allowed "platform-independent" software to be developed which could run on systems with different operating systems as well as different processors.

Java

More recently, the term "platform" has been overloaded to mean "application platform"--a (partially) generalized framework of user-interface, relational database and transaction processing, networking, security, development and deployment services within which a business application can be constructed. In this context, software is almost always written for a specific application platform; platform-independence at this level is rare.