What rules are tested during the Wisconsin Card Sorting Task? I was reading about the Wisconsin Card Sorting Test and although I understand the concept, I'm not clear on what rules are usually tested. Does it always vary between associations between shape, colour and number? For example, let's say I have a card with two red circles. Are the only possible matches to this card something with two shapes on it, red shapes on it or circles on it? Are more complex rules, such as the number of shapes + 1 or circles associate to green also tested?
If you read between the lines of Evaluation of a Short-Form of the Berg Card Sorting Test, you can find that indeed the only rules tested are the simple colour, number and shape matching. This can be identified by either looking at the source code attached to the paper or looking at the sample result table.

Update:

See also this paper.