In my surveys I use this (at first seemingly strange) contruction in order to allow the participant to chose which question groups he wants to answer.
The construction is as follows: For a certain combination of variables that are assessed earlier (1), I set a corresponding default combination of groups (2), which is presented to the participant as a multiple choice item (4). The default groups (2) are set via assignment in equation items (3) on the preceding page (because there is no way to fill in equations in the default item list of a multiple choice question). The user is then allowed to actively change the default groups and to check which groups he wants to answer. The data from this multiple choice item is later used directly as relevance of the corresponding question groups (5). If the group was ticked in the multiple choice item, it should be presented, i.e. be relevant.

(1) assess variables via different question items
(2) generate a default combination of question groups
(3) tick default groups in multiple choice item via assignment in equation item
(4) present the multiple choice item to the user, where he can change the choices
(5) set relevance of groups according to the choices in (4)