When using filenames (check out transaction FILE - client independant or SF01 - client dependant), you should also check out the standard SAP functionality for logical file paths and logical file names. It can be reached via transaction code FILE. A logical file path is a data record that contains a path name. A program would select the file path rather than use a hard-coded file path.
With a logical file path, the path can be set, and with the logical file name, the file name can be determined. SAP allows the usage of several variabele name settings, e.g. /urs/trans/<SYSID>/more can be used as logical file path, the string <SYSID> would be replaced by the actual system name.
Function module FILE_GET_NAME can be used to retrieve/compose a file name. File names are related to file paths and if it's only the path you need, access table PATH directly.
The actual function call to FILE_GET_NAME allows the caller to pass on two parameter fields: PARAMETER_1 and PARAMETER_2. These parameter values will replace the parameter from /urs/trans/<PARAM_1>/more.