Constant IMMUTABLE_EXTENSIONS 

const IMMUTABLE_EXTENSIONS: u64 = 0x0B;

Immutable Extensions, Extension Header Type 0xB.

Section 11.2: “The Immutable Extensions (Extension Header Type 0xB) contains a sequence of Key-Value-Pairs (see Figure 2) which are also Track or Object Extension Headers.” The Type is odd, so its value is length-prefixed bytes, and those bytes are another delta-typed run starting from 0.