Can you explain whether you are trying to accomplish something in particular or just trying to reverse engineer the formula to understand what's going on?
To answer your question, from what I can tell, Order Date Unified is basically taking the Year selected in the Today Parameter, and converting ALL dates to that year. For example, if Today Parameter = 1/31/13, then ALL order dates are converted to the same date, but the year would be 2013 (e.g., 1/31/10 now equals 1/31/13).
Sales YTD This Year = all sales from this year (based on the year of Today Parameter)
Sales YTD Previous Year = all sales from previous year (based on the year of Today Parameter)
Sales after YTD = all sales from all other years (so if Today Parameter = 1/31/12, sales from 2010 and 2013 would be in this category).