In reply to dave_59:
The FIFOs are in the uvm_in_order_class_comparator.
The handle for that class is the comp_cmd.
The function flush() is inherited from uvm_in_order_comparator, while the do_flush() is inherited from uvm_component.
This is the class hierarchy from uvm_in_order_class_comparator up to its parents:
uvm_object extends uvm_void
uvm_report_object extends uvm_object
uvm_component extends uvm_report_object
uvm_in_order_comparator #( type T = int , type comp_type = uvm_built_in_comp , type convert = uvm_built_in_converter , type pair_type = uvm_built_in_pair ) extends uvm_component
uvm_in_order_class_comparator extends uvm_in_order_comparator