Hi,
Rigid transform block is used to apply a time-invariant transformation between two frames. The transformation rotates and translates the follower port frame (F) with respect to the base port frame (B). In the example, the rigid transform blocks are connected in reverse. Connecting the frame ports in reverse causes the transformation itself to reverse.
Two rigid transform blocks, since two end frames are used in the example. If only one frame is used, then a single Rigid Transform block is sufficient. You can also refer to Rigid Transform MathWorks Documentation for future reference.
Hope it helps!