Module tex 

Core data structures, such as CategoryCodes and Tokens.

Modules

catcodes

Category codes

characters

Data structures for reading input text.

nodes

TeX Nodes that end up in the final document.

numerics

Numerical values used in TeX, such as integers and dimensions.

tokens

A Token is – conceptually – either a control sequence, or a pair of a character and a CategoryCode. In practice, we use CommandCode instead, which omits “impossible” codes (e.g. Invalid or Comment) and adds internal ones (e.g. Primitive or Argument).