With a base 10 place value number system, Britain and America use the period as the radix symbol, to separate integers and decimals, and use a comma to separate groups of digits; for example, they would write

3,200,100.56

Other Europeans such as the French, Italians, Spanish and Norwegians use the comma as a radix symbol and a period as a separator, so would write

3.200.100,56

Yet others like the Swedes and Finns use the comma as a radix symbol and use a small space as a separator, so would write

3 200 100,56

The Germans however would write

3 200.100,56

This can represent a problem for translation systems. For instance, internationally the following number is ambiguous

100.523

However, in all cases the underlying principle is that breaking long numbers, into groups of 3 or less digits, facilitates speed and accuracy when reading numbers.

The following earlier posts are relevant