The code number is the height of the rail measured in thousanths of an inch.
Therefore code 100 = 100/1000 inch high (one tenth of an inch). This scales out as 7.62" in 4mm scale.
Code 75 = 75/1000 inch high (Slightly more than one thirteenth of an inch high) This scales out at 5.7" in 4mm scale.
Perhaps someone can advise the actual height of prototype rail for comparison.