Your Contact Email Triggers Determine the Contact Stages Blocked By Default for Any New Sequence You Create
Within any Sequence Ruleset or the Settings of any Sequence not using a predefined ruleset, you have a configurable field to Mark emails as failed when user sets the contact stage to be any of: whichever Contact Stages you specify in that field.
If you create a new Sequence with a predefined ruleset, that setting is determined by whatever you have configured in that ruleset.
If you create a new Sequence without selecting a ruleset, Apollo applies some default Sequence Settings. These defaults include four stages we automatically add to the Mark emails as failed when user sets the contact stage to be any of: setting. Those four default contact stages are mapped directly from the stages you have configured in the following Contact Email Triggers, accessible under Settings > Contact (scroll down to Triggers):