From the website;
"An SRL-style goal list takes more work but can produce more balanced boards. The generator will try to use 1 goal from each difficulty tier and arrange them so that every row, column, and diagonal adds up to the same difficulty value. If 2 goals have one of the same "types" then it will refuse to put them in the same row. This lets you keep incompatible or broken goal pairs from appearing together."

I think the "difficulties" it's talking about are the "types".