In choosing a default email address under your Email Account settings, that determines which inbox (or sender email address) is the default sender for you if you (a) choose to send from 'Contact Owner' instead of a specific email address inside of your Sequences, or (b) choose to send yourself 'Send Tests' from your email templates (meaning they will be sent from that address when you initiate a 'send test').
Regarding sequences, and choosing which inbox to send from in your sequences, each sequence gives you the option to choose an email account to send from, or you can choose to send from Contact Owner. If you choose to send from Contact Owner, this is where the Default Email Address comes in. If you are on the growth Plan or higher, you may also use our Sending Distribution feature which allows you to send from multiple sender accounts in the same sequence to scale up your sends.