any one reply me with example iam new into ax
this - current object
elemet - Form run object
you can find more information in below
http://stackoverflow.com/questions/4260403/this-vs-element-keyword-in-x
https://dynamicsaxinsight.wordpress.com/2014/05/08/ax-2012-this-vs-element-when-to-use-what/
This has been discussed several times in the forum. You can do a search.