The Spectrum file format is one of the 2 line formats that the ACS-SASSI user interfaces can read and display. ( TimeHist is the other ). This file format will have many extensions (*.RS, *.TFU, …) but the format for the file is always the same for all the extensions
The Spectrum format is a table like file format. The first column of each line is the value of the X axis for all of the lines in the file. Each other column is the Y value of a line for X value in the first column.