The term codebase, or code base, is used in software development to mean the whole collection of source code used to build a particular application or component. Typically, the codebase includes only human-written source code files, and not, e.g., source code files generated by other tools or binary library files. However, it generally does include configuration and property files.
The codebase for a project is typically stored in a source control repository. A source code repository is a place where large amounts of source code are kept, either publicly or privately. They are often used by multi-developer projects to handle various versions and developers submitting various patches of code in an organized fashion. Subversion is a popular tool used to handle this workflow, and is common in open source projects.
Referring to multiple codebases as "distinct" declares that there are independent implementations without shared source code and that historically these implementations did not evolve from a common codebase. In the case of standards this may be a way of demonstrating interoperability by showing two independent pieces of software that implement a given standard.