Q:

What's so "special" about the two SPI chip select digital outputs?

In the majority of Pi SPI documentation, it makes reference to the fact that 2 SPI chip select channels exist.
However, from my previous experiencing with the Raspberry Pi and other microcontrollers,  SPI comms can be implemented using GPIO as chip select lines.
My question: Why does Pi documentation state there are two chip select lines? What's special about these digital outputs? Are they coupled directly to the SPI controller?

A:

The main SPI hardware has two preset chip selects.
The auxiliary SPI hardware has three preset chip selects.
You can ignore the presets and use your own GPIO, but you will have to switch them yourself. The hardware automatically switches the presets.
The current Linux driver makes no use of this anyhow. It switches the chip selects manually.  You can tell by examining the mode of the chosen chip selects.  At one time they would have been in mode ALT0 (SPI mode).  Now they will be in mode OUTPUT.

