You can segment your lists into sub-lists by going into your list that you want to split up, then click the 'Action' button and select 'Segment List'.
This dialog box will pop-up and ask you for a list prefix name, and number of segments. Ex. if you list has 100 contact, you could split it into 4 lists by giving a segment number of 4. This is useful if you don't want to send to your entire list at once or on the same day.
For example, if I had a Big List of 29 contacts. I enter a list segment prefix of 'Big List' and a Number of Segments of 3, the result would like this.