Hi @kartheek_a ,
When you select event type as 'Rule Based' you must select an event schema and its field on which you want to trigger a journey. This indicates, throughout the journey you only can make use of fields which are part of same event schema. It means one can't use other event schema fields in journey.
However you can use profile schema fields regardless of which schema they belongs to in journey for personalization. Thanks to union of profile schema feature. Hope this helps