When do I need new io_service?

I am recently working a lot with boost::asio and mulling over its documentation which is not of much help in some matters. I cannot wrap my head around a few questions which are associated with one another. The most important is this - when would I need to create my own io_service implementation (and how to do it properly)?

Let's take an example. I want to handle message bus communication with asio. The library that provides the API returns to me an opaque pointer to connection which allows me to only write requests and read responses. So far, so good. All these operations can be handled via boost::asio::post operation within any io_context really. Do I need a new io_service here to handle this? Or an io_object is sufficient here? Where to draw the line?

It is possible I don't really understand what these concepts (io_service and io_object) represent and this is the root cause of all the fuss