During slicing, you can choose how large you want your slices to be, or whether each slice is a Section, a Chapter or something else. These are called "Slice Levels".
After uploading your ebook file for slicing, Slicebooks identifies up to 3 slice level options. For ePub files, slice levels are based on how the file is tagged. Tags on ePub files are usually set at Section, Chapter and sometimes sub-chapter and even page levels.
Here's how it looks when tags are set at 2 levels. The user chose Level 2.
Commonly tags are set as follows:
Section - Level 1
Chapter - Level 2
Sub-Chapter - Level 3
Be sure to review each slice level before slicing your ebook to assure you receive what you want.