"Note that almost all applications require neither SIMPLEX_TX nor SIMPLEX_RX"

Documentation
1

That’s pretty vague… does it mean in can omit the mode entirely ? Or “Very few applications require SIMPLE_TX or SIMPLE_RX” ?

See “http://devwiki.electricimp.com/doku.php?id=electricimpapi:spi:configure”

2

This just means that if you’re doing normal bi-directional SPI, you don’t specify either SIMPLEX_TX or SIMPLEX_RX.

SIMPLEX = unidirectional
DUPLEX = bidirectional