Unlike the top-down and bottom-up approaches, which are a rather vertical type of modeling, the inside-out one is a horizontal approach. It consists in defining key processes in the overall process and then complementing them with other processes. This approach may be helpful to modelers in different areas in case none of the vertical approaches is appropriate. Like both other approaches, this one has its negatives too. One of them is that when using this approach, there might be difficulties in defining what the key processes are.
In the process composer, you can use the inside-out approach and represent the key processes in the overall process as subprocesses. You can represent a complex task as a subprocess if you want to focus on the task as one of the key processes in the model. You can later add to the subprocess some human and automated activities, which in that sense could act like additional small processes whose details are not important.