Mode 900 is a glitch in THPS3 PC. Not much known about the nature of this title, some may refer to the subject as "camera glitch".
This glitch allows player to enter free roam mode by pressing "change camera" button + "ollie" button. Skater now will stuck in its place (even in tha air of grind) and player will be able to control the camera using Ctrl + arrows for moving and Shift + arrows for rotation. However player can still control the skater, which was widely used to cheat. By pressing abovementioned combination again skater will be released, but camera will remain controlled by the player. By pressing combination again everything returns to its default state.
Probably that is one of developer's debug functions that for some reason wasn't removed from the game.
Often it is used to take screenshots that can't be normally taken in the game. May be used to easily beat scores in the career mode. Abusing this glitch online would normally lead to ban.