It really depends on how your flow is setup, but if it's without any logic/variables and a simple 'set assignment group', you could do it like this (but also check out on AWA/assignment rules).

var assGrp = '';
var reqType = fd_data.trigger.current.your_field; // find the request type field on your trigger or other step
if(reqType = 'A'){ // check if you need to set values or sys id's or anything else, depending on how your variable reqType is set
    assGrp = 'sys_id_of_your_group';
} else if (reqType = 'B'){
    assGrp = 'sys_id_of_your_other_group';
}
return assGrp;