Wooow!! Thank you very much for your help and recommendations! I really appreciatte it! I'm gonna visit some links you sent me and also, try to get some of the recommended books.
Gareth your response is more than clear, now I have a clearer idea of the XPath functionality, mainly at the time that I'm implementing certain functions with ACL in Arbortext. As always your help is more than useful. Thank you so much!