BCal help system: The Tutorial
The first group presented to us is Layer III. The page is split into two parts. The first part allows us to provide absolute a priori chronological information for the group boundaries, i.e. any temporal information we already know about the true calendar dates of these events. The second part allows us to define relative a priori chronological information for the groups, i.e. how this group relates to other groups.
We have no absolute a priori information about the true calendar dates of the boundary parameters for this group, so we should leave these as "No absolute chronological Information". We can express relative chronological relationships between groups by selecting one of the relationship options listed. "abutting earlier" should be selected, because we know that Layer III abuts and is earlier than Layer II.