Answer
This comes down to which control type the form uses: a cRecListDataField control (intended for master records) supports the full record-selector sorting/column-configuration behavior, while a cRecSelExtComboBox control (intended for detail records) doesn't carry that same level of record-selector support. If a customized form needs sortable/configurable record-selector columns, building it around a cRecListDataField-style control rather than cRecSelExtComboBox is what enables that.