4 letter code under cockpit rego markings

[NZ744]

Hi,

I apologise if them seems a silly question but what do the 4 letter codes under the aircraft registration markings in the flight deck represent?

Eg: "EP-AF" under the aircraft registration marking in this photo:

Photos: Boeing 777-346/ER Aircraft Pictures | Airliners.net

Cheers

[compressor stall]

SELCAL........

[NZ744]

Makes sense...thanks!