1, 2, and 3 indicate the presence or absence of a hole in the saddle, and what guides the choice is the rotation of the pelvis/flexibility of our body. For those who have excellent pelvis rotation, size 3 is recommended, while for others, it scales down to size 1.
1: indicates a closed saddle, meaning the absence of a hole.
2: indicates the presence of a smaller hole called "Flow."
3: indicates the presence of the hole called "Superflow" (the widest one).
People who are more "rigid" can use size 1, but also 2 and 3. On the contrary, it is not recommended for a flexible person to use a saddle without a hole.