One transfer a script expects, and what the part answers.
Static
The next transfer to the address fails, the way a missing or busy part does.
The driver reads from the address and receives reply, whose length is the length it must ask for.
reply
The driver writes exactly bytes to the address.
bytes
The driver writes bytes and then reads reply in one transaction, the shape of a register read.
One transfer a script expects, and what the part answers.