Sessions are always tied directly to a specific date on the calendar. Sessions are synonymous with the lesson being taught that week across the entire curriculum. There are always 13 sessions per quarter.
Collections are pre-built bundles of content that have been packaged for a specific teacher/leader/parent within a given Session. Collections are used differently based on the brand and age level of your curriculum.
For example, if you use Gospel Project for Kids, Collections are used to distinguish additional age-level classifications within Kids - such as Older Kids curriculum from Younger Kids curriculum. If you are using Explore the Bible Adult, Collections are used to distinguish translations - such as between CSB resources and KJV resources.