So Tango is the pheonetic for T - T will be short for target. This come from years of training before 9/11 when SF operators etc were working with targets on ranges more than they were pouring sand out of their boots. It has spread from there to normal temrinology.
X-ray, being X, is I AM PRETTY SURE refers to Other persons of interest, this could be civilians, unidnetified tango's or unknowns. In most situations that you see on telly, an Xray is the hostage, but this is to ignore the complications that we really encounter in real life where you have Tango's pretending to hide amongst hostages (like the 1980? Iranian Embassy Seige).
I am also PRETTY SURE that you will never refer to friendly forces (FF) as Xrays. The reason is simple, suppose your ten man swat team are attaking a builiding and you and Bob are supposed to go through the living room windows and then clear the kitchen. Lets say Bob get hits and you scream Xray down on the net, nobody knows who you are, who is hit, friendly or stranger or where you are. Combat nets are busy things. But if you scream bob is hit, then everyone knows you are still up and in the fght (cos you are with him and broadcasting) and tha bob is wounded/down in either the livin room or kitchen. This allows HQ/control to dispatch medical aid to the right place, other teams to support you etc.