Documentation changes:
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.

Example:
# doktype items before: 1,2,3,4,5
TCEFORM.pages.doktype {
  keepItems = 1,2,255
}
# doktype items after: 1,2