Constant REPOS_UNPLACED 

pub const REPOS_UNPLACED: u8 = 0xFF;

The inventory-grid position an item that has never been placed holds.

Repos_PosX and Repos_Posy are BYTEs read only by the loader for items inside a bag, and both default to 0xFF rather than to a plain zero. That loader drops both values, so the default is what the read takes rather than anything a position depends on.