Constant PORTRAIT_ID_USE_RESREF 

pub const PORTRAIT_ID_USE_RESREF: u16 = 0xFFFF;

The PortraitId value that hands control back to the Portrait resref.

A trigger’s read stamps this when the file carries no PortraitId, which lands inside the documented >= 0xFFFE range where the engine consults the string field instead of the id. Defaulting to 0 would name portrait row zero and shadow the resref, which is the opposite behaviour.