Update hook
Use this hook to (re) determine the value of the field based on the current record values. Think of determining defaults and calculating derived values.
Example:
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
}