some questions about latched relay driver

[james.qiu]

When I am reading the Keysight 34970 DAQ datasheet, it lists all the relay Multiplexer circuit. the relay used in Multiplexer board is latched relay.
the control signal is from MCU to 74HC574, the signal from 74hc574 to transistor base pin. the driver for coil is made by 2 PNP + 2 NPN transistor a H bridge.

https://testworld.com/wp-content/uploads/service-guide-keysight-agilent-data-acquisition-switch-unit.pdf

why add a 74hc574 between MCU output and transistor base?
why some relay drivers not use freewheeling diode, some drivers use freewheeling diode?
why some relay drivers use two freewheeling diodes?

driver no freewheeling diode page261

drivers use two freewheeling diodes on set relay direction. page242

 

[BradtheRad]

The latching relay closes by a pulse in one direction, and resets by a pulse in the other direction. So the coil may generate a spike in either direction. We mistakenly might try to solve this by putting two anti-parallel freewheeling diodes across it, but then they rob current from going through the relay coil.

It could be worth a try to install two back-to-back zener diodes, whose rating is slightly greater than the supply voltage. However it's unknown: a) how effectively zener diodes reduce spikes, b) how well they endure repeated spikes, and c) how well they protect neighboring components from over-voltage bursts.

Ordinary silicon diodes are robust and drop the voltage to 0.6 V.

I think the schematics depict different approaches using plain diodes, to solve the spike problem.