Assigning content to Everyone means both members and non-members will be able to access it.
Content assigned to Members Only will only be accessible by logged in members.
Making a subscription plan available for purchase by Everyone means current members and the public can join it.
Making a subscription plan available for purchase by New Members means only non-members can join it.
Making a subscription plan available for purchase by Existing Members means a signup must already be a member of your site.