Tournament Director for Windows implements the FIDE rating and ECF grading systems. The ECF grading is main grading system including the automatic submission of the results for ECF Grading.

• ECF Grading system
  This is an excellent explanation of the ECF Grading system and its workings.
   
• FIDE Rating system
  This is an explanation of the FIDE rating system
   

Here is additional background to the ELO system on which FIDE Rating system is based.

As an aside, here are the details of the Scottish grading system