There are 40 versions of the QR Code. Details of these are explained here.
iQR Code
QR Code Features, Use Cases, Custom QR Codes, QR Code Examples
Chapter 27. QR (2D-Barcode)
Developer Office Hours
What is a QR Code?
How many QR codes can be generated? - Quora
Calaméo - The Scoop on QR Codes
QR code - Wikipedia
Difference in capacity of QR code when calculating manually and when referring to online references - Stack Overflow