Custom text output allow to generate customized text files with information inside the report. For each expression you can define a Export Expression, formating with Export Display format.
Because the output will be a text file, you can specify the line number (Export Line), if you set this property to 0, the current line will be used. The Export Position property will place the text in the specified position inside the line, ans Export Size will specify the size of the text, blanks will be added to fill the size.
The property Export New Line, will generate a new line after exporting the element, current line is always the last line of the text. You can control the export order by using bring to front and send to back buttons at the designer, so when you define the last element of a line you should push the bring to front button, so it's exported the last of the current section.
See Expression reference for more information.