>An existiong appication has a form in which the "set the filter" property in the Data Enviroment to
>

=(csoitecode = gcsitecode)

>where csitecode is a field in the table and gcsitecode is a global variable. I used this format because it seems to work.
>
>In the valid method of a field it uses the seek function after using the file to as a different name to check if a value has already been entered for a site. Unless I add an explicti set filter" command for the aliased table it finds if a value is entered for other sites.
>
>Can some one please explain why?

It sounds like you are using the table twice. The filter will not work across areas