'Scheduling an Email' on a 'Contact Details' popout:
In this scenario, you are sending a 'one-on-one' email, under the contact details page. while doing so, you have the ability to select a specific 'date and time' under the email as highlighted in the screenshot below.
In doing so, a task will be created under your 'Sales Inbox' from where you will have the ability to open the task, read the email and make any edits if required before sending it out.
'Scheduling/Pausing an Email' under the 'Sequence Builder Step'.
In this scenario, you are sending out an 'Email' step via a sequence and you would prefer a human, an 'assigned user' added under the sequence settings to review, make edits, etc before sending out the email.
In turning 'ON' the 'Pause' button as highlighted in the screenshot below, the task will be created in the 'assigned users' - 'Sales Inbox' as per the sequence 'run time' settings. The user can then read, make any necessary changes if required and send out the email.
This step when used under the sequence, will not allow you the option to select the 'date and time' under the email. But, will send (create a task) as per the sequence run time settings.
Note: If a 'Scheduled Email' task is completed by the selected user under the 'Sales Inbox' outside of your sequence 'send time' settings, it will send out immediately.
In a nutshell, the difference between the two options is to be able to pick a date and time, if this is done inside the Contact Details pop out with a one-on-one email, whereas inside a sequence using the 'pause' button will create a task under the assigned users 'Sales Inbox; when the step runs as per the sequence 'Settings', and from there the email can be read and sent or edited before sending if required.