Sudoku Companions are boxes that join to a single junction box.
There are always four companion boxes associated with one junction box.
There are nine patterns which look something like the following:
Two companion isolation #1 | End | Next
It pays to try and solve as many missing squares in a Junction box as possible
by using the Companion boxes. Once you find as many as possible in one
Junction, move on to another until you've covered all nine Junction boxes.
Then repeat the cycle again, and again, until you don't find any more squares.
Another 5 must go in the center row after the 1, 2 in the right box (x).
Another 7 must go in the bottom row between the 5, 3 in the center box (x).
Another 9 must go in the bottom row before the 3 in the center box (x).
Another 2 must go in the top corner of the upper-right box (x).
Another 4 must go in the center row between the 1, 7 in the right box (x).
With a "what's left" list, you may be able to isolate an individual empty cell by two other forms of Exclusion. One form is when a missing number can only go into a single empty cell of a row or column because that number is blocked in all remaining empty cells. The other form is when a box has some (or none) of the missing numbers, and an empty cell in that box intersects with a row or column that contains the remaining missing numbers EXCEPT ONE. For example, if 2,3,6,7 are missing in R3, and the B2 block already contains both 2 and 6, and intersecting column C4 contains a 3, then you know 7 must go in the empty cell at C4 of R3.
The 6 can be found by the first form of exclusion, and the 7 can be found by the second form of exclusion. The 2 can then be found by the first form, and 3 is what's left.
Another way to look at this is to notice the 6 and 2 in B2, which means 6 and 2 must be outside that box, in B1 and B3. C9 contains a 6 forcing C9 to be 2, and C2 must be 6. Likewise, 3 and 7 must appear in B2, and C4 contains a 3 forcing C4 to be 7, and C6 must be 3. The "what's left list" (WLL) helps you to focus on just the missing numbers.