RB472142772CN Please help.

Hello!
please expalin me this, what is null semnification : RB472142772CN 2013 NULL PEK MD 20130617 0354
item No. Year Status Location Destination Country Date
RB472142772CN 2013 Collection 31203100 MD 20130525 1625
RB472142772CN 2013 Waiting for transit airline Space 31002400 MD 20130527 1351
RB472142772CN 2013 Dispatching 31002400 MD 20130527 1445
RB472142772CN 2013 Departure from China Beijing MD 20130616 0822
RB472142772CN 2013 NULL PEK MD 20130617 0354

thank you
all the best
Nadia