Cellular Automata rule function in mathematica / notebook source, Patterns, Alternatives - wolfram-mathematica

Question

I found this from a Cellular Automata Mathematica file, what is pattern and alternatives?
In this block of code, what does Pattern mean:
CellularAutomaton[{ { 0, Blank[], 3} -> 0,
{ Blank[], 2, 3} -> 3,
{ 1, 1, 3 } -> 4,
{ Blank[], 1, 4} -> 4,
{ Alternatives[1, 2] << 1 or 2 >>, 3, Blank[]} -> 5,
{ Pattern[$CellContext`p, Alternatives[0, 1]], 4, Blank[]} -> 7 - $CellContext`p,
{ 7, 2, 6} -> 3,
{ 7, Blank[], Blank[]} -> 7,
{ Blank[], 7, Pattern[$CellContext`p, Alternatives[1, 2]]} -> $CellContext`p,
{ Blank[], Pattern[$CellContext`p, Alternatives[5, 6]], Blank[]} -> 7 - $CellContext`p,
{ Alternatives[5, 6], Pattern[$CellContext`p, Alternatives[1, 2]], Blank[]} -> 7 - $CellContext`p,
{ Alternatives[5, 6], 0, 0} -> 1,
{ Blank[], Pattern[$CellContext`p, Alternatives[1, 2]], Blank[]} -> $CellContext`p,
{ Blank[], Blank[], Blank[]} -> 0}, {

Answer 1

You are defining a Cellular Automaton explicitly.
Each row defines an evolution rule.
You may find the relevant information here.
A few hints for reading your code:
Blank[] is the blank pattern "_" that matches any expression
Pattern[] is Mathematica construct for pattern matching
Alternatives[a,b,c] is the full form for a | b| c ... any of "a, b or c"
Pattern[p, Alternatives[a, b]] names as p the matched expr (a or b)
Edit
So, as an example, the following Automatons are made equivalent:
CellularAutomaton[{
{1, 0} -> 1,
{1, 1} -> 0,
{0, 1} -> 1,
{0, 0} -> 1},
{0, 1, 0}, 10]
CellularAutomaton[{
{1, Pattern[t, Alternatives[1, 0]]} -> Abs[1 - t],
{0, 1} -> 1,
{0, 0} -> 1
}, {0, 1, 0}, 10]
Note: The example is just for you to ease the understanding of the code you posted. There are better ways to define this Automaton.