The MCU is great, but the datasheet (in particular the SPI section) is very hard to read. Whatever the original language was, it wasn't English, and I guess Google translate has been involved. I've added an example below, and hope for ST to update the datasheet in the near future.
Example 1:
To understand better the next detailed content of this section, the user should capture the
configuration impact and meaning of the following items at first:
Data size (DSIZE) - defines data frame (sets the number of bits at single data frame).
FIFO threshold (FTHLV) - defines data packet, sets the number of data frames at single
data packet and so the occurrence of the packet occupancy events to handle SPI data
registers either by software or by DMA.
Data access – a way how to handle the SPI data register content when the transfer data
between the application and the SPI FIFOs upon a packet event. It depends on the packet
size configuration. Optionally, multiply data can be handled effectively by a single access of
the register (by data packing) or by sequence of such accesses (when servicing a bigger
data packet).
FIFO size – capacity or space to absorb available data. It depends on the data size and the
internal hardware efficiency how the data is compressed and organized within this space.
The FTHLV setting must respect the FIFO capacity to store two data packets at least.
Transaction size (TSIZE) – defines total number of data frames involved at a transaction
session overall possibly covered by several data packet services. There is no need to align
this number with the packet size (handling of a last not aligned data packet is supported if
TSIZE is programmed properly).
Example 2:
The setting of data communication format follows the basic principle that sure number of
data with a flexible size must be transferred within a session (transaction) while, optionally,
the data handling can be cumulated effectively into a single access of the SPI data registers
(data packing) or even grouped into a sequence of such services if data is collected at
consistent bigger data packets
