That means that most/many Pi Plates that plug into the Model B will plug into the B+ just fine. They wont sit in the same location - they'll be slid down just a bit but electrically-wise its the same.
There's also a pair of odd pins, ID_SD and ID_SC. The note says they are reserved for PiPlate ID EEPROM! What's that mean? Sounds like the Pi foundation took a hint from the BeagleBone Black design for BBB capes. The BBB capes all have a shared I2C bus for a classic 24LC type EEPROM. When the BBB boots, it reads the EEPROM and configures the bone inputs and outputs and kernel modules, etc. based on the EEPROM contents.
The ID pins are likely the same, its a secondary I2C port that looks just for these identifing EEPROMs. That way the Pi knows what Pi Plate is attached.