1) Assuming test bench top in your case is top level environment:
As in the top level environment, the sequence object is created, so you can access particular variable in top env like abject.variable.
sequence_h = sequence::type_id::create(sequence_h);
b = sequence_h.a;//variable in sequence
2) Assuming test bench top in your case is top level module:
In sequence if you have config variable "a".If you want to pass that to the top level module, then by using the uvc component you can do it.
env.agent.mon.a = this.a;
in Monitor the variavble "a " is connected to interface . so interface signals can be accessed at the top module.
tb_top.a = this.a // not advisible.
I think this helps,and if anyone knows better way or other ways please let me know.