# 3.2.2.1 Page start, symbol category descriptors

Bytes from 0x0000 to 0x059 contain signatures and the **SymbolCategory**
structures, as specified in section 2.6.1.1.1,
describing the symbols used for compression of data for the content
index key. Five symbol categories are described in the following
section. The first category contains symbols with values from 0 to 0x81, the
second category contains symbols from 0x82 to 0x103, and so on. The numbers of
bits used to store corresponding elements in the **OccCount bit stream **field
of **ExtensionDataPages** are 0, 3, 7, 10 and 24 for the first, second,
third, fourth and fifth categories, respectively. The number of bits used to
store the element for the first category is 0 because the value is not changed
from the previous document identifier.

**Figure 6: Page start, symbol category descriptors**