A standard RTM (Release to Manufacturing) ISO of Windows 8.1 from 2013 is missing over 500 security and reliability patches. An Extra Quality build has been "slipstreamed" using tools like NTLite or MSMG Toolkit. This means:
While each release varies, the following features are commonly advertised in high-quality custom ISOs:
Such labels often indicate modified ISOs. These can contain malware, spyware, or altered system files.
(for Windows 8.1)