Why are travel vaccines divided into routine, recommended and required?

This is the division of vaccination established by the World Health Organization (WHO).
Required vaccines: regulated by the WHO International Health Regulations (IHR), usually they are vaccines against yellow fever, poliomyelitis and meningococcal infection (vaccinations are required for pilgrims going to Mecca (Hajj, Umrah), in some countries - returning from this area).
Routine vaccines: are administered in accordance with the national legislation for specific age groups.
Recommended vaccinations: recommended depending on the specific country with regard to specific infectious diseases common in that area, individual risk of the traveller, etc.