Join Activity Format
Join is not actually an activity. Joins are a convention of outcomes, but are represented in the journey as activities to make them easier to edit. Multiple outcomes with the same next value, pointing to the same activity, constitutes a join. Joins are used by marketers to to easily join branches back together. This activity type cannot be used in a custom activity.
The activity must be formatted as specified below. The following JSON object is included in the activities array of a journey, as defined in the Journey Specification.