Custom Segments are segments you can build manually using email lists or by combining, refining, or excluding user sets from your existing segments. 

Custom List 

The Custom List starting point gives you the ability to create a list of guests based on email addresses or from a frozen segment. You can add guest emails manually or upload a CSV containing a list of guests emails to automatically populate your segment. Alternately, you can add guests using the current snapshot (frozen) of one of your segments to populate your custom list with the contents of that segment's guest list.

Custom List segment attributes

• Custom List (manual)
  Type email addresses for each of the guests, separated by a comma or a new line, in order to manually add them to the segment.

• Custom List (from CSV) 
  Click Browse to upload a CSV file containing email addresses to automatically populate your segment. 
  ^{There will be no immediate indication that the CSV file has been uploaded. Once you've selected it, click Save and Show. If you go back in and edit the segment, the emails from the CSV list you uploaded will now be shown in the Custom List field.}

^{The CSV file must be formatted with "Email" as the header text and each email address in its own cell in the same column, as seen below:}

• ^{Freeze from Segment 
  If you "Freeze from Segment" you are basically duplicating another segment's current guest list and using it to populate this new custom list segment. This option doesn't create a new frozen segment. Rather, it acts as a one-time import of another segment's contents.}