The language of the audio track. Castellano specifies that the movie is dubbed in Peninsular Spanish (the dialect spoken in Spain), as opposed to "Español Latino" (Latin American Spanish). EspaDivx.com
The site had a massive forum where users requested specific releases. A thread might read: La.vida.es.bella-DVDrip--Castellano--EspaDivx.com-
If you want a better experience than a 2009 DVDrip from a defunct pirate site, here are legal streaming options in Spain and Latin America (as of 2025): The language of the audio track