[GPS] Difference between xtra.bin and xtra2.bin

Hi everyone!
In gps.conf file, we have :

XTRA_SERVER_1=http://xtra1.gpsonextra.net/xtra.bin
XTRA_SERVER_2=http://xtra2.gpsonextra.net/xtra.bin
XTRA_SERVER_3=http://xtra3.gpsonextra.net/xtra.bin

but sometimes we have

XTRA_SERVER_1=http://xtra1.gpsonextra.net/xtra2.bin
XTRA_SERVER_2=http://xtra2.gpsonextra.net/xtra2.bin
XTRA_SERVER_3=http://xtra3.gpsonextra.net/xtra2.bin

What is the difference for our gps ?