PLT Scheme is one dialect of the Scheme programming language, and there are many others. Indeed, “Scheme” is perhaps more of an idea than a specific language.
The #lang prefix on modules is a particular feature of PLT Scheme, and programs that start with #lang are unlikely to run in other implementations of Scheme. At the same time, programs that do not start with #lang (or another PLT Scheme module form) do not work with the default mode of most PLT Scheme tools.
“PLT Scheme” is not, however, the only dialect of Scheme that is supported by PLT Scheme tools. On the contrary, PLT Scheme tools are designed to support multiple dialects of Scheme and even multiple languages, which allows the PLT Scheme tool suite to serve multiple communities. PLT Scheme also gives programmers and researchers the tools they need to explore and create new languages.