understanding I2CWrite optional address parameter?

[richard]

PCF8547p/t/a is an unusual situation having only one register to read or write from, the chip cares not how many reads or writes you squeeze into one transaction.
the final result is the last byte written or read.

all i2c read transactions start with a write transaction sequence to set the register to read from if necessary a PCF8547p/t/a ignores this action.

to make any sense of what you allege without the code to demonstrate it is near impossible