It is not necessary to use these room numbers - it is up to the programmer to decide, but if they are used then the obj.in.room command can be used to find out if the object is in the current room when the player tries to get it.
Room 255 is the player's inventory, so any inventory items the player is to have at the start of the game should have their room number set to this.
Note: Do not confuse inventory objects with screen objects. These are two completely different things. Inventory objects (or "inventory items") are things that the player carries round with them during the game. Screen objects are views that are displayed on screen and represent things like characters and props. All the commands in the logic programming language that are called (something).obj refer to screen objects, except for show.obj and show.obj.v. Usually when the word "object" is used in the logic programming documentation it means screen object.
You can edit the OBJECT file with the Object editor