Short answers, pulled from the story.
The Ratio system allows players to assign a numerical value from one to four to each character on their team. This mechanic replaced the rigid structure of the original 2000 release where teams were locked into fixed strengths. A team of three characters can be assigned a total ratio of four, meaning individual characters might possess less health and power than their single-character counterparts in the previous game.
The console versions for Dreamcast and PlayStation 2 arrived in Japan on the 13th of September 2001. This release occurred a month after the arcade debut on the NAOMI hardware. The PlayStation 2 version was later released in other regions without online support.
The arcade version initially featured 44 playable characters. The home console ports expanded the roster to 48, including exclusive characters that were not available in the arcade release. The home versions also included four bonus characters: Evil Ryu, Orochi Iori, Shin Akuma, and Ultimate Rugal.
The Groove system dictates special techniques and Super Gauge mechanics and includes four new play styles. Players can choose from six distinct Grooves, each with its own Subsystems such as dashes, running, and guard cancels. This system allows players to tailor their experience to their preferred fighting style.
In the true endings, Akuma or Rugal interrupts the celebration to demand a match against the winners on top of Osaka Castle. Depending on the player's Groove, one of two scenes plays where Akuma turns into Shin Akuma or Rugal transforms into God/Ultimate Rugal. The winning team then fights the bosses in the Osaka Ruins in Flames.