If you "measure" the bits per character in the base 45 alphanumeric encoding used in QR code, you'd get 5.5 bits per character as 11 bits is used for two characters.

How is it possible to have information less than a bit, a partial bit? What is that ".5" part? Isn't a bit indivisible?

Only in the context of a character doublet is all information expressed. To know the "half bit" part, you cannot "look" at just one character, you have to look at the total. The information is shared between the two characters. Measuring the bits-per-character is only useful when considering the whole system. The "partial bits" is information smeared across the system. Changing the middle bit may change one, or both, characters.

Here's a 11 bit example, where the middle bit is changed and it changes both characters: (11101001010 vs 11101101010, or '/L' vs '%8' encoded)

https://convert.zamicol.com/?in=11101001010&inAlpha=01&outAl...

https://convert.zamicol.com/?in=11101101010&inAlpha=01&outAl...

vs changing the last bit only changes the last character: (Using the preceding example, 11101101010 vs 11101101011, or '%8' vs '%9' encoded)

https://convert.zamicol.com/?in=11101101011&inAlpha=01&outAl...

The same principle applies to information theory and cryptography. Security can be measured in "partial bits" because it's measured across something larger.