what atlochowski said is true. "unlocked" means not carrier-locked/sim-locked.

most xperia X devices can be bootloader-unlocked. SOME cannot, including, for instance, docomo-branded devices from japan.

all you can do is buy one from someone that accepts returns. open the box, turn on the phone, open the phone-dialer-app, and dial *#*#7378423#*#*
if you see:
Service info > Configuration > Rooting Status
Bootloader unlock allowed: no

instead of "yes", return the phone.
if you see yes, then you can unlock it yourself fairly easily.

(ive bought one xperia-x, one xperia-xa2, and FIVE xperia-x-compacts, all from ebay in USA. only one of the compacts was allowed=no, the rest were allowed=yes)