`-bucketBy` can select to bucket by test 'class' or test 'case'

When set to 'class' dnd with a non-zero bucket size (N), tests will be
run in buckets of N test classes.  i.e. all test cases of a given class
will be run at once, in the same bucket.

When set to 'case' (the default) and with a non-zero bucket size (N),
all test cases will be split up into buckets of N test cases/methods.
It's possible that test cases from the same test class will be split
into separate buckets.