Dolby E offers the ability to carry up to 8 channels of audio on a single AES-3 digital audio channel complete with metadata. It is a digital audio compression technology designed for use by TV broadcast and production professionals in tv facilities. This is a convenient technology to reduce the number of cables needed to transport multiple audio channels around a facility. It also ensures phase coherent treatment of multiple related channels (e.g. 5.1 audio) and binds AC-3 encoded information (metadata) together with the audio.

Dolby Digital (AC-3) by comparison, is the encoding scheme used to transmit audio channels for surround sound and has no relationship to video.
Dolby E audio is frame locked to the video, allowing clean switching between video channels. Dolby E is also more robust and can better survive multiple generations of transcoding. The Dolby E encoder bundles the audio data into packets corresponding to each video frame. Dolby E is the standard transport format for surround sound in the production facility because it can be edited further. In comparison the much stronger compressed AC-3 stream is only intended for 'one-time use' - after decoding in the Receiver the audio data should not be processed or edited further. In the production process only in the last stage before mastering the audio stream is encoded to AC-3 (Dolby Digital).
Dolby E accomodates eight audio channels with soft compression, AC-3 (Dolby Digital) accomodates only 6 highly compressed audio channels.

If necessary to process video and audio separately, the Dolby E or AC-3 streams must be decoded back to the 4 or 3 separate AES audio streams.

Dolby E introduces a one frame delay latency to the signal path during the encoding process and one frame delay during the decoding.

Dolby Digital (AC-3) in comparison introduces approximately 5 frames of audio delay during the encoding process and one frame during the decoding process. It is designed for transmission to consumers and has high bandwidth efficiency and is not optimized for multiple encode/decode cycles or editing.
Dolby E allows programs to be decoded, processed and re-encoded many times without degradation. Audio and video frame rates are the same with Dolby E, enabling precise video picture edits without mutes or glitches.

Dolby E is used to get the audio to the transmitter and Dolby Digital to get the audio from the transmitter to the consumer.