What are the key differences and relationship between SCTE-104 and SCTE-35?

SCTE-104 messages can exist either in VANC space of baseband (SDI) video, or sent between systems through TCP/IP, and are typically used as a precursor to eventual creation of SCTE-35 messages.

SCTE-35 messages exist inside an MPEG Transport Stream, and contain PCR timing data and possibly component PID data specific to that transport stream. 

One way to think of this is that SCTE-104 messages are requests from an operator or automation system to create splice markers in a video stream, while SCTE-35 messages are the fulfillment of these requests, including all of the data that a downstream system will need to take appropriate action during live reception of the compressed stream.

The downstream system reading the SCTE-35 messages may be a server-side ad splicer, a set-top box, a video switcher, or any other equipment that can smoothly splice video from multiple sources into a single stream in the compressed domain. Based on the unique identifiers and flags in the SCTE-35 messages, the final operation may be insertion of an advertisement or other piece of interstitial video, overwriting of upstream video content with a locally generated slate, or segmentation of an in-progress recording into programs and chapters.