I believe it evaluates both and there is no 'preference'.
if either evaluates to TRUE it will suppress.
if you place a suppress condition in the DETAILS section it will sppress all of the sub sections below it (a, b and c in this case) when it evaluates to TRUE.
if you place it in the detailsA it will only suppress that sub-section.
IF DETAILS condition is TRUE and or DETAILS_A is FALSE, a is still suppressed.