Rule
Let's solve this puzzle with 3 columns and 3 rows.
Every square in the grid will contain a number 1-3. No number can repeat within any column or row.
The small number at the upper-left corner of a square is the total of the numbers in that block.
If a single number is designated to a block of a single square, that number will be assigned to that square.
Let's actually put numbers in the squares.
Put 1 in the top-right corner because it is a block of a single square.
Focus on the red block. The only combination of two numbers from 1-3 that will produce a total of 3 is {1,2}.
1 is already in the top-right square, so the top-left square will be 2.
1 and 2 are now put at the left column so the bottom-left square is automatically 3.
The center square in the bottom row can be determined to be 1 since 3 + Center square must be 4.
So the bottom square in the right-hand column must be 2.
Now, let's focus on this block. Center square at the right column must be 3, because 2 + Center square must be 5.
The same logic is used to complete the puzzle.
Done!
The order to place the numbers differs depending on which block you solve first. You can try solving the same puzzle again and again, starting each time with a different block.
