Fixed Point Notation is a representation of a fractional number as it is stored in memory.

The first digit of a binary number starts at 20 and increases by one power per digit after it. So if a decimal point was used the powers of the digits that follow it would decrease by 1 per extra digit past the decimal place

If we wanted to use 3 digits after the decimals point for example the first digit would be 2¹ the second digit would be 2² and the third digit would be 2³.if more digits past the decimal point need to be added the power of the digit would have to decrease by one for each digit.

For example if we wanted to display 25.625 in binary

1(2⁴)1(2³)0(2²)0(2¹)1(2⁰).1(2^-1)0(2^-2)1(2^-3)

So this would simplify as

16 +8 + 0 + 0 +1 + 0.5 +0 + 0.125

Equalling 25.625