On what basis does 'Evaluate new records only' in Journey evaluate new records added to DE?

Question

I have a journey and the sendable DE used for the journey looks like this,

EmailAddress
CUSTOMER_ID
Send_Date
sub_category
Unique_key - PRIMARY KEY

The automation runs everyday & appends new values to this above DE and triggers the journey. I have a few questions

1. If I use - Evaluate new records only in Journey Builder does that take only new records that were added /appended today to the DE?
2. I don't understand how SFMC evaluates the new records on what basis?

Example:

Say [email protected] is added to DE on day 1.
Again [email protected] is added to DE on day 2.

Will [email protected] considered as old or new record? (The second time this a@gmail has different primary key) I am confused.

Answer 1

From the official documentation:

When you set a schedule that repeats, you select a processing option.

• To evaluate only new contacts added since the previous run for journey entry, select Evaluate new records only. For the journey's first scheduled run, choose whether to evaluate all data extension records on only the first run or records added after activation. This evaluation option is typically used when a process exists to add new rows to the source data extension without deleting or overwriting existing rows. For example, the automation with an import activity appends records to a data extension. Using this option typically results in faster, more efficient processing.
• To evaluate all records in a data extension each time according to the entry source's schedule, select Evaluate all records. Use this option only when records in the data extension are continually updated. Using this option typically results in slower processing.

This evaluation is done based on the SubscriberKey field that you set in the Sendable Entry Source Data Extension for the Journey. In other words, when you have Append or Update data action methods in SQL Query or Import File Activities in Automation Studio to refresh data in your Entry Source Data Extension, Journey Builder compares whether SubscriberKey has been entered the journey before. If that is a new SubscriberKey then it can enter and there is no correlation to how much time passed or when the Data Extension was refreshed.

In your case, the "[email protected]" email address will be communicated a second time due to the new Primary Key (= SubscriberKey) and Journey Builder will view that as a new contact.