There's no universal grading system for wheat flour. But some countries do. As you correctly said in Italy 00 is the very fine flour from the early stages of milling. However, the same flour is marked as type 45 in France, Spain & Portual but in Germany it's type 405.
The general rule of thumb is, the larger the number the darker the flour, the lower the number the finer and whiter the flour is... but the numbering is not the same across countries. In fact I don't think either the US or the UK use any numbers at all.