Hi,

It sounds like you are confusing default values with reference qualifiers. A reference qualifier returns a "where clause" to the query. E.g. if you want only active users to show up in your field that reference sys_user, the reference qualifier is 'active=true' It is ALWAYS a query filter.

If you want it to be variable based on another field, then you need to construct that query filter using the other field.

For example: javascript:'location=' + current.requested_for.location

If it gets more advanced then that, I usually use a function my own script include and call it (known as an advanced reference qualifier.)

http://wiki.servicenow.com/index.php?title=Reference_Qualifiers