Pl see these MOS Docs
1377613.1 and 1189783.1
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In Place Upgrade
Upgrades will done as patch sets to Oracle Base (minimum patch set)
Out of Place Upgrade
Upgrade will be done outside the Oracle base, and will be directly upgrades without any minimum patch sets (this is the Oracle recommended approach)
Note: Out of Place upgrade is introduced only from 220.127.116.11 database patchset.
In addition please also see the following links:
Hope this helps!