The string is a specific file naming convention used by the website GateAnime . It refers to: GateAnime.com : The hosting/distribution site. AoT : Attack on Titan (Shingeki no Kyojin). TFS : The Final Season . P3 : Part 3 (The Final Chapters). 01 : Episode 1 (or the first special).
The finale of Attack on Titan was split into two long-form specials rather than a traditional weekly series. This allowed Studio MAPPA to maintain high production quality for the complex "Battle of Heaven and Earth." MAPPA Director: Yuichiro Hayashi Theme Music: "Under the Tree" by SiM Viewer Guide gateanime-com-aot-tfs-p3-01-480md-mp4
Optimized for smaller screens where high resolution isn't as noticeable. The string is a specific file naming convention