Add Column > Conditional Column builds an if-then-else rule from drop-downs: pick a column, an operator and a value, then the output. Add Clause adds another "else if" line, and the Else box catches every row that matched no clause.
Clauses are tested top to bottom and the first match wins. That makes the order important for ranges: test "Qty is greater than or equal to 12" before "greater than or equal to 6", or every large order is labeled Medium.
Behind the dialog, Power Query writes if ... then ... else if ... else ... in M, which you can read in the formula bar.