What is decimal notation?

Answer 1

its 5.5 rather than 5 and a half (or 5 1/2)

It means that you represent fractions of numbers in tenths (and hundredths, thousandths, etc) rather than in fractions (or in eighths or twelfths or some such).

the above notation would be expected in the US. However, in Europe, the notation would be 5,5 for 5 and a half. Basically, the Europeans reverse the use of comma and period in their decimal notation.

Answer 2

Now try and remember. When did you last see it? What were you doing at the time?