What does the layer param flag PF_ParamFlag_LAYER_PARAM_IS_TRACKMATTE do exactly?

AE_Effect.h wrote:

/* only valid for layer parameters. indicates that a layer param is used as a track-matte with applied filters (used in Premiere, ignored in AE) */PF_ParamFlag_LAYER_PARAM_IS_TRACKMATTE			= PF_ParamFlag_USE_VALUE_FOR_OLD_PROJECTS,

I applied this flag to a layer parameter, and thought that after the user specifying the layer parameter, the selected layer would act like a track matte layer in the track matte effect (invisible under normal circumstances).

However, this flag seemed to mess up the checked out layer data instead.

So, my question is: what exactly does this flag do?