Extra is available for sector, hittype, sprite, and wall structs.
For sprites, extra typically the actor-in-question's health value. It's also used to store the damage of hard-coded projectiles. See also htextra.
For a sector, extra contains the value of any GSPEED sprites that were in the sector (Gspeed sprites are deleted when the level loads up)
For a wall, extra is reset to -1 at map load time in-game because the game uses the field for internal purposes. You can access the values before they're wiped in EVENT_PRELEVEL, however.