Use the processor's pull requirement to only pull the type of box it accepts.
I seem to have had a bad translation earlier and interpreted the question as the need to create different flowitems which is not what you're asking. I'll leave that answer here in case it's of any use:
To switch object created - use the FlowItemClass property. Here's an example with a FlowItem Column in a schedule:

On reset:
Table schedule=current.attrs.variables.subnodes["schedule"];
string nextFlowItemClass=schedule[1]["FlowItem"];
current.setProperty("FlowItemClass",nextFlowItemClass);Then on creation:
Table schedule=current.attrs.variables.subnodes["schedule"];
int repeatSchedule=current.attrs.variables.subnodes["repeat"].value;
if (rownumber<schedule.numRows || repeatSchedule) {
if (rownumber==schedule.numRows)
rownumber=0;
string nextFlowItemClass=schedule[rownumber+1]["FlowItem"];
current.setProperty("FlowItemClass",nextFlowItemClass);
}