It's a spin-off series that follows the more comedic aspects of the original Full Metal Panic! It focuses on the daily life of Sousuke Sagara and Kaname Chidori, with a much lighter tone compared to the original series.
There are two other anime series that follow Full Metal Panic? Fumoffu:
* Full Metal Panic! The Second Raid: This series picks up after the events of the original Full Metal Panic! and focuses on the more serious aspects of Sousuke's life as a soldier.
* Full Metal Panic! Invisible Victory: This series is the latest installment in the franchise, focusing on Sousuke's continued adventures as a soldier.
It's worth noting that the series is not strictly a sequel in the traditional sense, but rather a continuation of the overall Full Metal Panic! universe. Each series has its own distinct tone and focus, but all contribute to the overarching story.