Hi kconllin,
1. If your campaigns means customer journey,
we should firstly know that customer journey's final target(marketing email recipients) can only be contacts,
because whatever default entity we set in segment, it will finally query all related contacts with this entity,
it's by design.
2. Contact's Originating Lead and Marketing only could be fields/flags which helps us identify the type of contacts in marketing module,
Originating Lead is only selectable when create new contact and it will be blocked once the contact has been saved.
By default Marketing only is set to No.
I will list scenarios for identification:
> 01 When you create a contact in CRM directly in CRM/Marketing
Contact:
Originating Lead: Null
Marketing only: No
> 02 When you qualify a lead to contact
Contact:
Originating Lead: originate lead
Marketing only: No
Lead:
Existing Contact?: the auto-generated contact
> 03 When recipient creates a new record on marketing page.
Contact:
Originating Lead: Null
Marketing only: Yes
Lead:
Existing Contact?: the auto-generated contact
> 04 When recipient updates his/her record on marketing page.
It will only update contact's fields on marketing form.
----------------------------------------------------------------------
So, when you build a segment based on Lead:
Find Marketing leads:

Find Marketing qualified leads:

Find Sales qualified leads:

Find leads generated from Marketing page:

Please read following doc of how to work with marketing to lead.
https://docs.microsoft.com/en-us/dynamics365/customer-engagement/marketing/market-to-leads#keep-automatically-generated-contacts-from-confusing-users
Regards,
Clofly