By default, any sub-collections within a Collection Tree are sorted by modification date. This default can be changed on the level of individual Collection Trees. Collection Trees can be sorted alphabetically, by size, or by date created or modified, in ascending or descending order. You can also rearrange them by drag-and-drop. The sort order you select applies to display in the left-hand side panel as well as in the Table of Contents view, if you have it enabled for a given collection.