Hearing aids continue to develop and change at a fast rate but one thing that doesn't change is the capability of the microphone. The telecoil has been the main function, since about the mid seventies, that has allowed hearing aid wearers and people with cochlear implants to listen to induction loops in a wide range of places where they have difficulty in hearing especially over distance, background noise and areas with poor acoustics.
There are hearing aids now that because of their small profile manufacturers have not included the telecoil or have made a decision not to include the telecoil in some of their hearing aids. How do people, with that type of hearing aid, have access to speech, sound, telephone, TV, mobile phone and some even have a telecoil to listen where there is an induction loop? They have produced their own devices that are now generically know as "Streamers". In place of a telecoil, in the hearing aids, there is a receiver which receives transmitted sound from the Streamer. The Streamer can be wirelessly connected to other devices the manufacturer has produced for microphones, mobile phones, landline telephones, TV, audio, Laptops, iPads to name a few.
Please remember that a manufacturers equipment will only work with their hearing aids. If you did want to use other manufactures equipment the only way, at the moment, would be if the Streamer has a Direct Audio Input. This allows you to plug in the other manufacturers universal miniature receiver. Two examples would be connecting the Phonak Roger X to the Streamer and using a Roger pen or connecting Digisystem DT20 and using a DM10 Microphone.
Why would you want to mix manufacturers? It may be situational, education or the workplace, where the Streamer Microphone only works over a shorter range and the other manufacturers equipment works over a longer range and provides more options.