Fields can be marked as Required for planners by selecting the particular field and enabling/disabling the option: Required for planners.
This means that when planners update that specific field (e.g. in the event or participant details), the field becomes marked red.
Please note that even when this option is enabled for a specific Participant field, this setting does not make this field mandatory for participants (e.g. something they must submit in a form). It simply means that event planners will have to provide this piece of information somehow. If you want to make a field required for participants, this needs to be set up in the Forms builder.