The C refers to the program being written in the C programming language as opposed to Fortran in which TranSIESTA was written. This code had been completely reengineered and further developed into the commercial products marketed by the company today.
The C code represents the current version of the program, distinct from the Fortran-based TranSIESTA that preceded it. This code has undergone substantial modifications and enhancements, resulting in the commercial software products currently marketed by the company.