where is HPSM search variable $choices define?

Hi Experts,

I'm trying to remove the "assigned to me" and "high priority" option in the OOB dynamic search as shown.

I notice that it is be created by the variable $choices, whenever the $L.tablename (look for table name field) changes, I had look in Format control, link but could not find any thing that actually define the value of the variable $choices. Can some one please advice where can I find it? 

Value from RAD debugger

>d $choices
<form><checkbox id="open" label="Open"/><checkbox id="closed" label="Closed"/><checkbox id="assigned" label="Assigned to me"/><checkbox id="highpriority" label="High Priority"/></form>

My objective is to remove the last two option.

 

Regards

Soong