To ensure that older clients do not access data incorrectly, Bazaar’s policy is to introduce a new storage format whenever new features requiring new metadata are added. New storage formats may also be introduced to improve performance and scalability.
The newest format, 2a, is highly recommended. If your project is not using 2a, then you should suggest to the project owner to upgrade.
Some of the older formats have two variants: a plain one and a rich-root one. The latter include an additional field about the root of the tree. There is no performance cost for using a rich-root format but you cannot easily merge changes from a rich-root format into a plain format. As a consequence, moving a project to a rich-root format takes some co-ordination in that all contributors need to upgrade their repositories around the same time. 2a and all future formats will be implicitly rich-root.