Question on show ip interface brief

[wfqk]

Hi I used command show ip interface brief. and I got the following output. My question is why Port G1/20 has TFTP, while other ports have unset ?

Thank you

SW-(config)#do show ip interface brief

GigabitEthernet1/18    unassigned      YES unset  down                  down    
GigabitEthernet1/19    unassigned      YES unset  down                  down    
GigabitEthernet1/20    unassigned      YES TFTP   down                  down    
GigabitEthernet1/21    unassigned      YES unset  down                  down