From the CRBasic version 3.4 Help file: “TDF stands for Table Definitions File. When a program is compiled for a CR1000, CR3000, or CR800/850 datalogger a program_name.TDF file is created along with the original program file. This file contains the table definitions (table size, variable names, data types, etc.) for that program. In software that works with this functionality, the user can associate a TDF file with a data logger. This can be useful if communication is taking place over a slow or unreliable communications link where the attempt to receive table definitions back from the data logger fails.”