chargeable

In bsnl usage webpage, wat does that "chargeable usage" mean? im having a home 500 plan, where i ve 2.5 gb free a month. right now, i ve around 600 mb in chargeable usage. does this chargeable usage mean i ve exhausted my 2.5 gb n i ve to pay more? or it is within my 2.5 gb limit, n i can use...