This command "Mark as a library element" exists in Modeling with ModelManager, but not with Windchill.
Yet, after migration from ModelManager to Windchill, the "marked as a library element" parts keep this function.
It is very helpful when an assembly is copied with "One Level" option not checked: a new assembly with new parts is created, but all the screws, nuts, ... (all part "marked as a library element") are not copied, but shared.
Remark: a part "marked as a library element" keeps this function even when Modeling is not linked with
ModelManager or Windchill. It seems that this function is a Modeling function, no a database function.
The idea is to create a command in Modeling "Mark as a library element". It is not necessary that it is linked with Windchill.