For the first release, a TREP 3 license is required and TREP will need to be installed at the customer site. Through next year we will at the capability to our hosted offerings such as Elektron Connect and Elektron as a Service. These will not need a TREP license explicitly but a contract for the relevant TR service will be needed.
This question was first asked in an Introduction to the Elektron WebSocket API webinar (29th of Nov). Please see here: Introduction to the Elektron WebSocket API