While you could nest an activity into another manually, the Subprocess macro will come in handy.

The Track() function will snap the output to the next task that is “magnetic to” the track’s semantic.

While you could program this little piece of logic and flow yourself using a bunch of Ruby methods along with a considerable amount of ifs and elses, and maybe elsif, if you’re feeling fancy, a Trailblazer activity provides you a simple API for creating such flow without having to write and maintain any control code. It is an abstraction.