Subtests are test sets linked to test results. It is easier to understand this by looking at a simple example.
1..3 ok 1 - First test 1..2 ok 1 - This is a subtest ok 2 - So is this 1..2 ok 1 - This is a subtest ok 2 - So is this ok 2 - An example subtest ok 3 - Third test
In the example above, we have a test set with three tests. But the first test has another test set, with two tests. This is an example of subtests.
This feature has been added in tap4j version 3.0.