Is Derived hook

Use this hook to indicate whether the field is derived. If a field is derived, then the field is made read-only in the UI. The difference with the <cdf field>.is.readonly() hook is that the field value can be changed within other hooks of the extension, for example in the <cdf field>.update() hook. If a field is read-only, its value cannot be changed.

Example:

function extern boolean tcmcs004.cdf_addr.is.derived(long mode)
{
    if tcmcs004.crou(1;1) = "U" then
           return(true)
    endif
    return(false)
}
function extern tcmcs004.cdf_addr.update(long mode)
{
    if tcmcs004.crou(1;1) = "U" then
           tcmcs004.cdf_addr = tcmcs004.cdf_zip & " " & tcmcs004.cdf_city
    endif
}