AIUI, the "main" cup is the one from which detergent is released for the
"wash" phase, rather than the one from which detergent is released for the
"pre-wash" phase. The cup for the pre-wash phase may be smaller than the
other; it may not have a cover at all; or it may have a cover with holes
in it.

On my 8 -- 10 year old GE dishwasher "Main detergent cup for all wash
cycles" is embossed in the plastic door liner with an arrow pointing at the
detergent recess with a cover.

SJF