@warnerrs, you’re right in your observation but wrong in your conclusion :-). It is not the tools that generate these numbers but UVM itself. The number is a UVM object instance id, accessible via the uvm_object::get_inst_id(). For every uvm_object that UVM creates, it increments the ID. Therefore it will remain constant as long at you don’t change either your code (creating new objects, changing the order of creation), or your simulator (which might trigger changes to order of execution anyway).