The ID drop-down list in the Expression Builder dialog box (Edit | Expression) lists the set of items available based on the expression element type selected in the Expression Element Type drop-down list.
When Functions & Operators is chosen from the Expression Element Type drop-down list, the ID drop-down list contains a list of the available functions and operators.
When Features is chosen in the Expression Element Type drop-down list, the ID drop-down list displays the IDs of the features in the measurement routine.