There are two standards that define common practices for creating DCPs. The SMPTE standard and the Interop standard.
‘Interop’ wasn’t released as an official standard. The specifications for Interop DCPs are a collection of recommendations, based on a Digital Cinema Specification Document, released by the MPEG Interop Initiative in 2003. In this document, norms for the creation of digital cinema content were established for the first time ever.
The (newer) SMPTE standard was developed and released by the Digital Cinema Initiatives, LLC (DCI) and was meant to replace the older Interop standard.
Though the SMPTE standard allows for more flexibility (e.g. in regards to framerates and audio formats) today’s reality still is this: Not all cinema servers are equipped to play SMPTE DCPs properly. Years of experience have shown us: If you want to make sure your DCP works everywhere, the InterOp standard is the safest option available.
Not everything that is possible in a DCP from a technical perspective is also commonly used in digital cinemas. The simple reason behind this is that not every digital cinema server that was installed before the SMPTE standard existed can be updated with the ability to interpret SMPTE DCPs correctly. And – even if they can be updated – not every cinema does so.
For more information regarding the industry’s move towards SMPTE: https://www.smptedcp.com/