The Big Test is an unofficial test for command suitability given by Admiral Stahl. The test consists of Admiral Stahl placing a midshipman that had been pre-selected to spend their final year of initial officer training on board his ship (either the USS Devastator or USS Dominator) in command as an Acting Captain.
As a part of the test, the Acting Captain will encounter situations that someone who has gone through all the steps in becoming a Captain is expected to deal with. Most of those situations are planned as a part of the test such as unavailability of certain supplies, insubordination, interaction with other officers both above and below their acting rank, initiative within the confines of vague orders. The test also includes dealing with the unexpected such as combat, or situations where combat is only one of the alternatives etc.
Those that pass the Big Test with Outstanding grades are not only expected to eventually get their own command - likely much earlier than most others in their midshipmen graduating class, but are likely to find themselves in command of one of the elite ships of the Union Navy. The rest that pass the Big Test are also likely to eventually command Capital Ships, but would not receive Admiral Stahl's endorsement to be a captain of one of the elite ships.
Failing the Big Test can have a number of consequences ranging from the midshipman getting a "Not recommended for command of a ship larger than ..." "Not recommended for command of a Combat Ship", "Not recommended for command (at this time)" note by Admiral Stahl on their permanent record to having their career end at the moment their Big Test ended. While even those who get a "not recommended for command" note as a consequence of their failing the Big Test might eventually receive a command, the journey they would have to undergo to reach command is a lot tougher than what those who didn't get the opportunity to take the Big Test would have to achieve.
A gross failure of the Big Test will limit the promotion opportunities of the testee for the rest of their career - if not bring it to an immediate end.