What is the difference between MibSPI and SPI?
From the Reference Document:

This reference guide provides the specifications for a 16-bit
configurable synchronous multi-buffer serial peripheral interface
(MibSPI). The MibSPI is, in effect, a programmable-length shift
register used for high speed communication between external
peripherals or other microcontrollers. Its multi-buffer allows
multiple transmissions with different peripherals without any CPU
action.

It kind of is for your µc SPI communication what DMA is for memory, it allows you to offload some work from the CPU.