The The Revised6 Report on the Algorithmic Language Scheme defines a dialect of Scheme. We use R6RS to refer to both the standard and the language defined by the standard.
R6RS defines both libraries and top-level programs. Both correspond to PLT Scheme modules (see Modules). That is, although R6RS defines top-level programs as entry points, you can just as easily treat a library as an entry point when using PLT Scheme. The only difference is that an R6RS top-level program cannot export any bindings to other modules.