A hidden master surrogate's semantics are not shared, although sharing is used behind the scenes to avoid overhead.

Use hidden masters to lazy evaluate expensive operations. Clients often copy an area and then modify the copy (rather than the original). However, area-modifying operations frequently need to make a copy of that copy while performing their operations. By using surrogates, you can avoid copying the copy. Instead, the modifying operation creates and modifies another surrogate. It then processes the master objects of both surrogates, and installs the operation result as the master object of the target surrogate.

NOTE This technique results in a significant reduction of work and you should use it where expensive operations can be delayed or eliminated. Several of the books in the Bibliography discuss this technique.