C:\Users\ich\Documents\Arduino\hardware\esp8266\2.3.0-rc2\libraries\SPI
with SPI.h
class SPIClass {
public:
SPIClass();
void begin();
void end();
void setHwCs(bool use);
void setBitOrder(uint8_t bitOrder);
void setDataMode(uint8_t dataMode);
void setFrequency(uint32_t freq);
void setClockDivider(uint32_t clockDiv);
void beginTransaction(SPISettings settings);
uint8_t transfer(uint8_t data);
uint16_t transfer16(uint16_t data);
...
You see only one Argument in transfer() ..