This function basically corresponds to the mapping function LookupGlobalValue. However, the key is queried for the value and LookupGlobalKey gets an additional Key and the Key-Description, as parameters which are used to create new table fields when the key is not found.
LookupGlobalKey('LookupTabellenNamen','Key', 'Description')
LookupGlobalKey('LookupTabellenNamen','Key', 'Description', 'Value', 'Description')
LookupGlobalKey('currencies','5','Country code Switzerland')
LookupGlobalKey('currencies','5','Country code Switzerland', 'SFr', 'Currency Switzerland')