What do the mouths of A and B in Feng Shui mean?

Your statement is not quite right. It is not A, B, and Mao, but A, Mao, and Y.

Jia Mao and Yi are oriental trees. Among the twenty-four mountains, due east is the earthquake. Zhen is a hexagram in the Bagua. One hexagram governs three mountains, that is, the three mountains of Jia Mao and Yi.

In Feng Shui, *** is divided into twenty-four directions, each controlled by a hexagram, with three mountains (directions) in each hexagram.