Insert in doc_core_tsconfig, Page TSconfig, section TCEFORM_confObj, before property "removeItems":
Property:
keepItems
Data type:
list of values
Description:
Reduces a given set of elements until only the items defined in keepItems will be part of that set.
This property only performs a reduction and does not add items, thus elements which are present in keepItems but not in the original set of items won't be added.