Short answers, pulled from the story.
The AS-203 mission tested liquid hydrogen behavior in weightlessness to verify that fuel lines could maintain proper temperatures for engine restart. Engineers used this modified S-IVB second stage to prepare lunar missions to boost astronauts toward the Moon.
Technicians began erecting the booster at Pad 37B on the 19th of April 1966. The decision to launch AS-203 before AS-202 came in the spring of 1966 due to delays with the command and service module.
The tank was equipped with 88 sensors and two TV cameras to record the fuel's behavior. These instruments monitored every movement of the cryogenic fluid while the vehicle orbited Earth four times.
A closed fuel tank experiment involved pressurizing the hydrogen tank while depressurizing the oxygen tank until the pressure difference collapsed the common bulkhead separating them. This failure mode occurred during a ground test prior to launch and caused the rupture between the Manned Spacecraft Center and the Trinidad tracking station.
NASA classified the mission as a success because all primary objectives had been achieved before the explosion. Douglas Aircraft Company declared the design ready for use in September 1966, allowing engineers to proceed with sending men to the Moon using the validated restartable concept.