A Question about MLS in SELinux

["zhao hui" <zhaohui1976@xxxxxxxxx>]

In the book of <SELinux by Example>,in the chapter 8, there is a sentence says "

In recent enhancements to SELinux, the constraint feature has been extended to implement an optional multilevel security (MLS) policy. MLS is another form of mandatory access control, which is built upon type enforcement (TE). In this chapter, we explore the optional MLS policy features.

What does the "MLS is built upon type enforcement (TE)" means?

Thanks a lot