What are the differences between Rigid LATCH (UAS) and Flexible LATCH (UAS)?

Rigid LATCH (UAS)

The car seat connects to the vehicle’s lower anchorage system via a “fixed” or “rigid” bracket designed as part of the seat frame.

Flexible LATCH (UAS)

The car seat connects to the vehicle’s lower anchorage system via a “flexible” strap which routes through the seat's appropriate belt path.