John Arena writing in Pizza Today:

Generally speaking Italian flour is classified by how finely it is milled, meaning the degree to which the husk has been removed. A designation such as “00” will not tell you the protein content. This type of flour will feel more powdery regardless of actual grain size because it contains less of the course husk. There are type 00 flours with a wide range of protein levels for different applications. Usually protein content is the first thing that pizza makers look for in selecting a flour. Consider fermentation time when selecting flour. Longer fermentation will usually require higher protein.