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.
See Dialects of Scheme for general information about different dialects of Scheme within PLT Scheme.
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.