what does int'l mean when i checked my text usage on the phone?

[nueaey]

i'm using t-mobile phone and live in US.
i started to use twitter from yesterday, and people said "if you have unlimited texting, tmobile won't charge you if you text to twitter"..so i texted to twitter once, and checked my balance & text usage in case. and my balance was the same, but my text usage said "$0.60 Int'l"
what does that mean?
international? whag about "$0.60" part? did tmobile charge me or what? 'cause i wanna keep texting to twitter but if theyre gonna charge me, then i can't.

[widemint967]

International.