Copied for American Iris Society Site.

4. Border Bearded (BB) -- essentially small versions of the TBs in the same height range and bloom size as the intermediates, but blooming with the tall beardeds. Good BBs have round, ruffled petals that complement their small size.

5. Miniature Tall Bearded (MTB) -- this class is distinguished by daintiness and delicacy. Height from 41 cm (16 inches) to 70 cm (27 1/2 inches). The blooms are smaller than on a BB and the stems are thin and wiry. An MTB clump looks like a cloud of butterflies. They are often called "Table Irises" because they are so well suited for arrangements.

You can read about all of the bearded classes as well as the more popular beardless types at the AIS page linked below:

Here is a link that might be useful: iris classifications

Thank you both

Carla