The constants (*.tlo\*.clo) file is a LCP output file, which is used as input for EMTDC to set-up the transmission line interface to the greater electric network. This file contains all the information necessary for EMTDC to represent the transmission segment in the time domain, and is for the most part, of no concern to the user.
By default, this file is automatically generated by the LCP, and will overwrite any file of the same name that already exists in the temporary folder if the corresponding segment has been modified. However, the user may also opt to provide their own custom file. See Overhead Line Configuration or Cable Configuration for more details.