The definition of unit testing from Wikipedia is:

In computer programming, unit testing is a procedure used to validate that individual modules or units of source code are working properly.

For Tester this means that a single unit would be a code entry in the test list, but you can define the size of a code entry to be whatever you want. There are no rules on what you must or mustn't do.

Each entry is simply a block of code that is first LOAD'ed and then DO'ed. If the block returns a result, the result is stored for display in the result list in the main window.

The latest version of Tester will check against an intended result, and report back whether the result is as intended, which of course is part of unit testing.

But real unit testing should offer totally independent tests, and this is where Tester is different, since Tester focuses on testing many parts sequentially and keeping track of results during the test. Furthermore, Tester is not yet able to completely separate each test run, since the tests are done in the same REBOL session as Tester exists in.

If you want to compare Tester to anything, you can consider the REBOL console a hand cranked version of Tester.