These two features are very similar but have important differences. It is generally considered that they both exist due to the convergence of Blackboard 8 and Web CT. Multiple Random Blocks or Question Sets can be added to a single test.
These are created by selecting a pool and/or other test and choosing the number of questions that the test randomly selects from to ask students.
This is a more granular version of Random blocks where you can select specific questions from pools and/or other tests that the test randomly selects from to display to students.